Problem: Engine Hesitation

Hesitation is when your engine misfires, stumbles or lacks power when you accelerate or step on the throttle. The problem often means the air/fuel mixture is not being properly enriched or is going lean, or the ignition system is weak and is misfiring when the engine comes under load or the air/fuel mixture goes lean. When you step down on the accelerator and the throttle opens, the engine sucks in more air. The computer should respond by adding more fuel.


If the engine has a speed-density type of fuel injection system (no airflow sensor), the computer uses inputs from the throttle position sensor, manifold absolute pressure sensor, air temperature sensor and engine rpm to estimate airflow and how much fuel the engine needs. NOTE: Speed-density systems are much less sensitive to vacuum leaks than EFI systems that use an airflow sensor. If the engine has an airflow sensor (vane airflow or mass airflow), it looks primarily at the airflow signal from the airflow sensor, but also takes into account what the throttle position sensor and MAP sensor (if equipped) are telling it. NOTE: Airflow EFI systems are very sensitive to vacuum leaks, and air leaks downstream of the airflow sensor. Consequently, if the inputs from any of these sensors is inaccurate or missing, the engine computer may not add enough fuel, allowing the fuel mixture to go lean causing a misfire that produces a hesitation or stumble when accelerating or opening the throttle. The amount of fuel added by the computer when the throttle opens may also be insufficient if the fuel injectors are dirty or fuel pressure is low. The oxygen sensors in the exhaust monitor the air/fuel mixture so the computer can adjust fuel trim as needed to maintain the proper air/fuel ratio. Fuel trim adjustments can compensate for dirty injectors and/or low fuel pressure to a certain extent, but occur too slowly to offset a throttle hesitation problem. Vacuum leaks will typically cause the fuel trim to run rich as the computer tries to compensate for the extra air being sucked into the engine through the leak. Possible Causes of Engine Hesitation or Stumble: * Dirty fuel injectors (cleaning the injectors often fixes this). * Bad MAP (manifold absolute pressure) sensor * Bad TPS (throttle position) sensor * Bad or dirty MAF (mass airflow) sensor * Low fuel pressure (leaky fuel pressure regulator, weak fuel pump or low system voltage or charging voltage that causes the fuel pump to run slow) * Vacuum leaks (intake manifold, vacuum hoses, throttle body, EGR valve) * Bad gasoline (fuel contaminated with water or too much alcohol) Sometimes, what feels like a hesitation is actually ignition misfirerather than lean misfire. The causes of ignition misfire may include: * Dirty or worn spark plugs * Bad plug wires * Weak ignition coil * Wet plug wires

A scan tool that can display sensor data and fuel trim values can help you diagnose a hesitation problem.

DIAGNOSE ENGINE HESITATION PROBLEM

Diagnose may require checking the engine computer with a scan tool for any fault codes (including misfire codes), checking sensor response with a scan tool by looking at the various sensor PIDS (sensor values displayed on the scan tool), and testing sensors with a DVOM or scope if the values are out of range or the sensor is not responding normally. Additional diagnostic checks may include searching for vacuum leaks, inspecting/cleaning the EGR valve, measuring fuel pressure and volume, removing and inspecting the spark plugs, etc.

You can use a scan tool to check the fuel trim readings to see if the engine is running lean. Lean mixtures that are caused by vacuum leaks will have the most noticeable effect at idle. At part and full throttle, there is so much air entering the engine that a little extra air from a vacuum leak has a negligible effect.

Plug your scan tool into the diagnostic connector and start the engine, then look at the Short Term Fuel Trim (STFT) and Long Term Fuel Trim (LTFT) values. Normal range is typically plus or minus 8. If the numbers are +10 or higher for STFT and LTFT, the engine is running LEAN. If you rev the engine to 1500 to 2000 rpm and hold it for a minute or so, and the STFT value drops back down to a more normal reading, it confirms the engine has a vacuum leak at idle. If the STFT value does not change much, the lean fuel condition is more likely a fuel delivery problem (weak fuel pump, restricted fuel filter, dirty fuel injectors or a leaky fuel pressure regulator) than a vacuum leak. For more information about using fuel trim to diagnose a lean fuel condition, read this article on Fuel Trim by Wells Manufacturing (PDF file, requires Adobe Acrobat to read). DIAGNOSTIC TIPS FOR P0171 OR P0174 LEAN TROUBLE CODES A code P0171 or P0174 (or both) indicate the engine is running lean. This means there is too much air and/or not enough fuel. If you have a scan tool that can display Short Term Fuel Trim (STFT), you can confirm the engine is running lean by looking at the STFT at idle. If STFT is greater than about 10 to 12, the engine is running LEAN. Increase engine speed to 1600 to 2000 rpm and hold for a minute or so, then recheck the STFT value. If it has dropped 3 or 4 points or more, the lean problem is due to a vacuum leak (vacuum leaks have more of an impact on the idle fuel mixture than the cruise mixture). If the STFT valve is still the same, the problem is probably sensor-related (dirty or bad MAF sensor, or bad MAP sensor), or is due to low fuel pressure. The next steps would be to use the scan tool to look at the airflow and MAP sensor readings, and to check fuel pressure. A lean fuel condition can be caused by: * Low fuel pressure due to a weak pump or leaky fuel pressure regulator. (use a fuel pressure gauge to check fuel pressure at idle) * Dirty fuel injectors. (try cleaning the injectors) * Vacuum leaks at the intake manifold, vacuum hose connections or throttle body. (Check for vacuum leaks) * Leaky EGR valve. (Check the operation of the EGR valve) * Leaky PCV Valve or hose. (Check valve and hose connections) * Dirty or defective Mass Airflow Sensor (MAF). (Try cleaning the MAF sensor wires or filament with aerosol electronics cleaner. Do NOT use anything else to clean the sensor, and do not touch the sensor wires) * TIP: On many Fords, a P0171 and/or P0174 Lean Code may sometimes appear because of a bad Differential Pressure Sensor (DPFE). This sensor monitors EGR flow, and is located on the engine near the EGR valve. There are two hoses that connect the sensor to the tube that runs from the exhaust manifold to the EGR valve. The sensor misreads EGR flow and the computer increases EGR which has a leaning effect on the fuel mixture. The fix is to replace the DPFE sensor. * TIP: For information about other causes of P0171 and P0174 lean codes on Ford Windstar, Click Here REPAIRS Repairs will depend on what is causing the hesitation. If the cause is fuel related, it may require cleaning the fuel injectors or fixing a vacuum leak. If the cause is ignition related, it may require replacing the spark plugs and plug wires.

Poor Fuel Economy

Is your vehicle delivering poor fuel economy? Have you noticed a gradual or sudden drop in the mileage you were once getting? The following are common causes of poor fuel economy that may or may not turn on your Check Engine Light or cause a loss of fuel economy:
Sluggish Oxygen Sensors

The oxygen sensors on your engine monitor the air/fuel mixture so the powertrain control module can add or subtract fuel as needed to meet changing operating conditions. As Oxygen sensors age, they become less responsive to changes in the air/fuel mixture, and typically produce a lean-bias signal. This tells the engine computer to add more fuel, when in fact the engine really doesn't need the extra fuel. The end result is a richer than normal fuel mixture that increases fuel consumption.

The fix here is to use a scan tool and/or digital storage oscilloscope to test the response of the oxygen sensors. Or, if your vehicle has a lot of miles on it (over 100,000) to simply replace the O2 sensors if you suspect they are getting sluggish.

You can also use a scan tool to look at Long Term Fuel Trim (LTFT). If the value is negative, it means the engine is running rich. This confirms the engine is wasting fuel, but it does not tell you why the engine is running rich. It could be sluggish oxygen sensors or some of the following causes.

 Inaccurate or Defective Coolant Sensor

The coolant sensor monitors the operating temperature of the coolant that is circulating inside the engine. If the sensor is defective and reads lower than normal, or always reads cold, the engine computer will keep operating in "open" loop - which means the fuel mixture remains rich. A richer fuel mixture is required while a cold engine is warming up to prevent it from stalling. But if the mixture remains rich once the engine is warm, it wastes the extra fuel and causes poor fuel economy.

The quickest way to check a coolant sensor is to plug in a scan tool and compare the coolant sensor reading with the inlet air temperature sensor reading when the engine is cold. But should show the same temperature reading. Then start the engine, and look for the coolant sensor to show a gradually increasing reading. If the engine eventually reaches 185 to 195 degrees ( once they warm up), the coolant sensor is probably okay.

If the coolant sensor reading does not change, or never reaches normal operating temperature, the problem could be the sensor or it could be a defective thermostat that is not closing when the engine is cold.

The next step would be to check the sensor's resistance reading with ah ohmmeter. If the reading does not match specifications for a given temperature, the sensor is bad and need to be replaced. If the sensor reads good, the problem is likely the engine thermostat.

Defective Engine Thermostat

The thermostat controls the operating temperature of the engine, and it helps the engine warm up quickly after a cold start. The thermostat is usually located in a housing where the upper radiator hose connects to the engine. When the engine is cold, the thermostat closes to block the flow of coolant. When a cold engine is started, the thermostat should remain closed until the coolant gets hot (around 185 to 195 degrees). If the thermostat does not close tightly or does not close at all, coolant will be circulating while the engine is trying to warm up. This will prevent the engine from warming up quickly, and it may never reach normal operating temperature. This can delay the powertrain control module from going into closed loop operation, causing a rich fuel mixture and poor fuel economy.

A quick check for this problem is to feel the upper radiator hose while the engine is warming up. If you feel coolant circulating through the hose following a cold start, the thermostat is probably stuck open. The fix is to replace the thermostat.

Engine Misfire

If an engine is misfiring for any reason, it will waste a LOT of fuel and result in poor fuel economy. Misfires can be caused by ignition problems such as worn or fouled spark plugs, bad plug wires, weak ignition coils or arcing between the plug wires or coil and ground. Misfires can also be caused by dirty or defective fuel injectors, vacuum leaks in the intake manifold, or low fuel pressure. Misfires can also be caused by loss of compression in one or more cylinders.

On 1996 and newer vehicles with OBD II, misfires should turn on the Check Engine Light and set a misfire code if the misfires are severe enough to cause an emissions problem. However, if the misfire rate is just below the threshold where a code must be set, you won't get a code or Check Engine light. If you have access to a factory scan tool or a professional level scan tool that can read something called "Mode $06" data, you can look at the actual misfire rates for each of the cylinders. from this, you can see if one or more cylinders are misfiring (even if they have not yet set a code).

If you do have a Check Engine Light and a cylinder specific misfire code (such as P0301 which would indicate cylinder #1 is misfiring), inspect the spark plug, plug wire (if used) and coil for that cylinder. If the ignition components appear to be working normally (no fouling, no shorting or arcing), the problem is likely a dirty or dead fuel injector.

If you get a P0300 "random misfire" code, the most likely cause is a lean fuel mixture due to an intake manifold vacuum leak, leaky EGR valve, or low fuel pressure.

Intake Manifold or EGR Valve Leak

A vacuum leak at the intake manifold gasket, in the manifold itself or any of its vacuum hose connections can lean out the air/fuel mixture and cause the engine to misfire and deliver poor fuel economy. Likewise, an EGR valve that does not close at idle, when the engine is cold or when it is not under load can allow exhaust to leak back into the intake manifold. This can also have a leaning effect and cause fuel-wasting misfires and poor fuel economy.

You need to check for vacuum leaks, and/or remove and clean the bottom of the EGR valve. Vacuum leaks can be found by spraying throttle cleaner along the edges of the intake manifold while the engine is idling. If the idle suddenly dhanges, it means some of the cleaner is being pulled into the engine through a leak. The fix usually requires replacing the intake manifold gasket, or the manifold itself if it is cracked. A cheaper fix is to apply a high temperature epoxy sealer to the crack and hope it seals the leak.

NOTE: It does not take much of a leak to upset the air/fuel ratio. Even a very small leak can cause problems. Professional technicians often use a device called a "smoke machine" to find small leaks. The machine generates a mineral vapor smoke, which is fed into the manifold (engine off). If there are any leaks, you will see the smoke seeping through the crack.

If no vacuum leaks are found, remove the EGR valve and check the underside of the valve and the port in the intake manifold for carbon deposits that may be preventing the valve from closing. Also, check the EGR valve's vacuum connections and solenoid to see if they are operating properly. There should be NO vacuum reaching the valve at idle or when the engine is cold.

Worn or Fouled Spark Plugs

Worn or fouled spark plugs will obviously cause fuel-wasting engine misfires. Platinum and Iridium plugs should last 100,000 miles, but short trip stop-and-go driving may cause the plugs to foul prematurely. An engine that is using oil can also foul out its spark plugs.

Remove and inspect the spark plugs. Clean the plugs if they are dirty, and regap to specifications, or better yet, just install a new set of spark plugs.

Dirty Fuel Injectors

Fuel varnish deposits can build up inside fuel injectors, preventing them from delivering their normal dose of fuel. This can cause a lean air/fuel mixture that results in lean misfires and wasted fuel.

Try adding a bottle of good quality (not the cheapest stuff) fuel injection cleaner to your fuel tank. It may take several tankfulls before any improvement is noticed. If that does not work, having the injectors professionally cleaned will often restore normal performance. If an injector is too badly clogged to be cleaned, or it is defective, you're looking at replacing one or more fuel injectors (which aren't cheap to replace!).

 Low Compression

If you are driving a high mileage vehicle (over 100,000 miles), you may be getting poor fuel economy because your engine does not have the compression it once had. As the miles add up, so does the wear on the piston rings and valves. This can result in a gradual loss of compression that reduces engine efficiency and fuel economy.

If you suspect low compression, do a compression test on the engine. If low, there is no easy fix other than an overhaul. There's no miracle cure in a can that will restore lost compression.

Wrong Oil Viscosity

Most late model passenger car engines today require a low viscosity 5W-20 or 5W-30 motor oil. Some even specify 0W-20. Such oils improve fuel economy, especially during cold weather when the oil tends to thicken. If you are using a heavier viscosity motor oil, it can reduce your fuel economy (maybe 5 to 10 percent depending on what you are using).

Dirty Air Filter

If your air filter is really dirty, it will interfere with normal engine breathing and hurt fuel economy. Remove and inspect the filter, and if it is dirty replace it with a new one.

 Clogged Converter or Exhaust Restriction

Any obstructions in the exhaust system will create power-robbing backpressure that also hurts fuel economy. You can inspect the outside of the system for any obvious signs of damage such as a crushed or crimped pipe. But internal problems such as a clogged converter or collapsed muffler or double walled pipe can't be seen from the outside.

You can check for an exhaust restriction by connecting a vacuum gauge to the intake manifold. At idle, the engine should show a high and steady vacuum reading (say 18 inches or higher). If the reading is less than this or it gradually drops, you have an exhaust restriction.

 Slipping Clutch or Transmission

If the clutch on a manual transmission is slipping, or the bands or torque converter lockup on an automatic transmission are slipping, some of the engine's power will be lost before it can reach the wheels. This can cause a noticeable drop in fuel economy -- and be VERY expensive to fix because it will require replacing the clutch or transmission.

 Low Tires

To achieve maximum fuel economy, your tires must be inflated to the recommended pressure for your vehicle and load. For most passenger car tires, that means 32 to 34 PSI. A low tire increases rolling resistance (and tire wear), and can result in a loss of 5 to 10 percent fuel economy. Check all four tires (when cold) with an accurate gauge, and inflate as needed to the recommended pressure.

 Dragging Brakes

A parking brake that is not fully releasing, or a brake caliper that is sticking can cause the brakes to drag and your engine to waste fuel. A quick check for this kind of problem is to park your vehicle on a slight incline, put the transmission in neutral, then release the brake pedal. If your car does not start to roll immediately, the brakes may be dragging.

 Too Much Junk in Your Trunk

More weight equals less fuel economy. It takes power to move mass, so if you are hauling a lot of unnecessary weight in the trunk or cargo area of your vehicle, you are not going to achieve maximum fuel economy.

 Poor Driving Habits

This is probably the most common problem and biggest fuel waster of all. Aggressive driving and jack rabbit starts flood the engine with extra fuel. Take it easy, as if you were driving with a raw egg under the gas pedal and you'll get the most miles per gallon from the fuel in your tank.

Fuel Trim

Fuel Trim is the adjustment the engine computer (PCM) makes to the fuel mixture to maintain a balanced air/fuel ratio. Fuel trim is usually displayed as a PERCENTAGE reading on a scan tool. For lowest emissions, the engine computer tries to keep the fuel mixture balanced around 14.7 to 1 (14.7 parts of air to one part fuel). If the air/fuel ratio is less than 14.7 to one (say 12 to 1), the fuel mixture is RICH. A rich fuel mixture can produce more power (up to a point) but it also increases fuel consumption and emissions. Conversely, if the fuel mixture is greater than 14.7 to one (say 16 to one), it is LEAN. A lean fuel mixture reduces fuel consumption but can also increase emissions if the air/fuel mixture is so lean that it fails to ignite and causes lean misfire.

The engine computer monitors the air/fuel ratio via the oxygen sensor(s) in the exhaust manifold(s). An oxygen sensor is essentially a RICH or LEAN indicator. When the engine is running lean (too much air and not enough fuel), the O2 sensor generates a low voltage signal that tells the engine computer more fuel is needed. When the engine is running rich (too much fuel and not enough air), the O2 sensor produces a higher voltage signal that tells the engine computer the engine is getting too much fuel and to cut back the fuel delivery. On vehicles that have an Wide Ratio Air/Fuel sensor (WRAF) or A/F sensor, the sensor tells the computer the exact air fuel sensor so the computer can increase or decrease the fuel delivery as needed. Accurate fuel trim values require an accurate feedback signal from the Oxygen sensor, otherwise the engine computer has no way of knowing whether the fuel mixture is running rich or lean. When a cold engine is first started, it may take 10 to 30 seconds or more for the heaters inside the oxygen sensors to warms the sensors up to operating temperature. Until that point is reached and the fuel feedback control system goes into "closed loop", the fuel mixture is fixed at a predetermined value so no fuel trim adjustments are made. But once the Oxygen sensors are hot and the coolant temperature is high enough for the computer to go into closed loop, the computer starts to generate fuel trim values and make adjustments in the fuel mixture. When the engine is shut off, the fuel trim values are retained in the computer�s memory so the next time the vehicle is driven it can pick up where it left off. Erasing the computer�s memory with a scan tool or by disconnecting the battery or the PCM power supply to clear codes also wipes the fuel trim values, which means the computer has to start learning the fuel adjustments all over again the next time the engine runs.

How to Read Fuel Trim

The fuel trim value is read by plugging a scan tool into the OBD II diagnostic connector located under the instrument panel (on the drivers side near the steering column). When the key is turned on, the scan tool will initialize and start to communicate with the vehicle�s onboard computer. Depending on the tool and the vehicle, it may be necessary to enter the vehicle year, make, model and engine VIN code before the scan tool can read the data.

The engine must be started and running to read the fuel trim information. Depending on the scan tool and how its menu options are set up, you choose the option that allows you to read system live data. This will display a long list of sensor outputs and other readings called PIDs (Parameter IDs). On this list will be two fuel trim values for inline four and six cylinder engines, and four fuel trim values for V6 and V8 engines (one pair for each cylinder bank).

There are two types of fuel trim values shown:

Short Term Fuel Trim (STFT) is what the engine computer is doing to the fuel mixture right now.

This value changes rapidly and can bounce around quite a bit depending on engine load, speed, temperature and other operating conditions).
Values normally range from negative 10 percent to positive 10 percent, though the readings may jump as much as 25 percent or more in either direction.

Long Term Fuel Trim (LTFT) is a longer term average of what the engine computer has been doing to balance the fuel mixture over a predetermined interval of time.
This value is a more accurate indicator of how the fuel mixture is being corrected to compensate for changes in the air/fuel ratio that are occurring inside the engine.

STFT B1 is Short Term Fuel Trim engine cylinder Bank 1
STFT B2 is Short Term Fuel Trim engine cylinder Bank 2
LTFT B1 is Long Term Fuel Trim engine cylinder Bank 1
LTFT B2 is Long Term Fuel Trim engine cylinder Bank 2

How do you know which cylinder bank is 1 or 2 on a V6 or V8 engine? Bank 1 will be the cylinder bank that has cylinder number one in the engine firing order. For more information on firing orders, see the following:
Firing Orders (Chevy)
Firing Orders (Chrysler)
Firing Orders (Ford)

What Fuel Trim Values Mean

POSITIVE fuel trim values mean the engine computer is adding fuel (increasing the pulse width or on-time of the fuel injectors) to add more fuel to the engine. In other words, it is attempting to RICHEN the fuel mixture because it thinks the engine�s air/fuel mixture is running too lean.

NEGATIVE (-) fuel trim values mean the engine computer is subtracting fuel (decreasing the pulse width or on-time of the fuel injectors) to reduce the amount of fuel injected into the engine. This is done to LEAN out the fuel mixture to compensate for what it perceives as a rich running condition.

Remember, all this is based on what the oxygen sensors are telling the engine computer. If the O2 sensors indicate LEAN, the computer adds fuel and generates a POSITIVE fuel trim value. If the O2 sensors are reading RICH, the computer compensates by subtracting fuel and generates a NEGATIVE fuel trim value.

By reading the STFT and LTFT fuel trim values on a scan tool while your engine is running, you can tell if the air/fuel mixture is running rich (negative fuel trim percentages) or lean (positive fuel trim percentages).

What Fuel Trim Values Should Be

Ideally, the STFT and LTFT should be within a few percentage points of zero when the engine is idling or being held at a steady RPM. Remember, STFT can bounce around quite a bit as when you suddenly snap open the throttle or decelerate. But LTFT can tell you if the average fuel/mixture is running rich or lean.

Good LTFT values should be as close to zero as possible, though they can range from 5 to 8 percent depending on the condition of the engine. If the LTFT is getting up around 10 percent or higher, it usually indicates a problem that needs to be diagnosed.

LTFT values that get up around 20 to 25 percent will usually set a P0171 or P0174 lean code.

LTFT values that drop down to negative 20 to 25 will usually set a P0172 or P0175 rich code.

This scantool is displaying a STFT value of 25 percent. Normally that would indicate a problem,
but in this case the engine is not running (Engine RPM is zero). As soon as the engine starts
and goes into closed loop, the fuel trim readings will begin to change.

How Fuel, Ignition and Engine Problems Affect Fuel Trim

Lean fuel mixtures are a more common problem than rich fuel mixtures, though either can happen depending on the cause.
LEAN fuel mixtures will generate higher than normal POSITIVE fuel trim readings on your scan tool.
RICH fuel mixtures will generate NEGATIVE fuel trim values.

Some possible causes of LEAN fuel mixtures include:

Air or vacuum leaks in the intake manifold, near the throttle body or at vacuum hose connections.

Weak fuel pump that is not generating enough pressure or volume

Fuel line restrictions (like a pinches hose or plugged filter)

A weak fuel pressure regulator that is not maintaining adequate fuel pressure

Air leaks in the PCV plumbing

Dirty MAF (Mass Airflow) sensor that is under reading airflow into the engine

Dirty or dead fuel injectors

Ignition misfire (a fouled spark plug, weak ignition coil or bad plug wire that causes a misfire allows unburned oxygen to pass into the exhaust and fool the O2 sensors)

Compression leaks (bad exhaust valve that allows unburned oxygen into exhaust and fools O2 sensors)

Exhaust manifold crack or gasket leak (allows unburned air into exhaust and fools O2 sensors)

Bad O2 sensor (signal shorted to ground so the sensor reads lean all the time)

Some possible causes of RICH fuel mixtures include:

Leaky fuel injector

Excessive fuel pressure due to bad fuel pressure regulator or restricted fuel return line

Extremely dirty air filter or restrictions in air intake system

Exhaust restrictions (clogged converter, crushed exhaust pipe or plugged muffler)

Bad O2 sensor (output shorted to voltage so it reads RICH all the time)

Using Fuel Trim to Diagnose Problems

Use Fuel Trim to Diagnose Vacuum and Fuel Delivery Leaks. With the engine idling, look at the Short Term Fuel Trim (STFT) and Long Term Fuel Trim (LTFT) values. Normal range may be high as plus or minus 8, but closer to zero is best. If the numbers are +10 or higher for STFT and LTFT, your engine is running LEAN. Rev the engine to 1500 to 2000 RPM and hold it steady for half a minute or so. If the fuel trim numbers drops back down to a more normal reading, it confirms the engine has a vacuum leak at idle. This is because vacuum leaks have less of a leaning effect on the fuel mixture as engine speed and load increase.

If the fuel trim readings do not change much, the lean fuel condition is more likely due to a fuel delivery problem (weak fuel pump, restricted fuel filter, dirty fuel injectors or a leaky fuel pressure regulator) than a vacuum leak.

LTFT fuel trim readings that are trending high might also be the result of a slight ignition misfire that is not bad enough yet to set a misfire code but is bad enough to cause a drop in fuel economy. One or more fouled spark plugs that are misfiring occasionally, or a weak ignition coil or bad plug wire that is allowing some occasional misfires could be the cause. For more information on misfire diagnose, Click Here.

You can use fuel trim to identify dirty fuel injectors. If the LTFT fuel trim readings are trending up (POSITIVE), it means the fuel feedback control system is compensating for an air/fuel mixture that is becoming progressively leaner over time. The most likely cause would be dirty fuel injectors. Fuel delivery can be restricted by the accumulation of varnish deposits inside the injector nozzles. The fix here is to clean the injectors. If the fuel trim values return to normal after the injectors have been cleaned, it verifies you have solved the problem. If the fuel trim values don't change after cleaning the injectors, the lean fuel condition may be due to low fuel pressure or air/vacuum leaks.

You can use fuel trim readings to check the response of the oxygen sensors and engine computer to changes you make in the fuel mixture. While the engine is idling, temporarily disconnect a vacuum hose. You should see the STFT fuel trim readings jump immediately and go POSITIVE, and the LTFT should start to creep up in response to the artificial lean fuel mixture you have just created by disconnecting the vacuum hose.

To test a rich response, you can feed some propane vapor from a small propane tank into the throttle body or a vacuum hose connection on the intake manifold. This time, you should see a drop in fuel trim readings, with STFT going NEGATIVE, and LTFT creeping downward in response to the rich fuel mixture.

No change in fuel trim readings when you create an artificial lean or rich fuel mixture would tell you the engine computer is NOT operating in closes loop, or that the oxygen sensor(s) are not responding to changes in the fuel mixture.

How Fuel Injection Affects Emissions

Fuel injection provides a continuously adjustable air/fuel mixture that changes with engine speed, load, throttle position, air temperature, coolant temperature and other operating conditions. This allows EFI to provide an Air/Fuel (A/F) ratio that minimizes emissions while maximizing fuel economy.

The optimum air/fuel ratio for lowest emissions is 14.7 pounds of air to 1 pound of fuel. This is called the "stoichiometric" ratio (or on European vehicles as Lambda equals one). An A/F ratio of 14.7 to 1 is chemically balanced and results in the most complete combustion with minimal carbon monoxide (CO) and unburned hydrocarbons (HC). Oxides of nitrogen (NOx) can still be relatively high depending on combustion temperatures, but at an A/F ratio of 14.7 to 1 a three-way catalytic converter will break down most of the NOx in the exhaust. The end result is almost zero HC and CO coming out of the tailpipe, and very little NOx.

The flexibility of a fuel injection system allows it to rebalance the A/F mixture as the engine's operating conditions change. The engine computer is preprogrammed with a basic fuel mixture map that includes various combinations of engine speed, load, temperature (air and coolant), throttle position and airflow. The sensor data that is most important for making A/F mixture adjustments depends on the type of EFI system.

BASIC EFI SYSTEMS There are two basic types of EFI systems: mass airflow and speed/density. With mass airflow systems, some type of airflow sensor is used to measure the volume of air entering the engine. On most late model vehicles, a hot wire Mass AirFlow (MAF) sensor is used. A MAF sensor reads airflow by varying the current needed to heat a wire that is cooled by air flowing past the wire. On many older vehicles (1990s vintage), a spring-loaded vane airflow sensor (VAF) was used to sense airflow. Toyota has also used a "vortex" airflow sensor on some models that detected turbulence in the airflow to indicate flow. The airflow sensor is located in the tubing between the air cleaner housing and throttle. The MAF sensor's reading is combined with other sensor inputs such as engine RPM, throttle position, engine load (via a Manifold Absolute Pressure sensor) and other information to calculate what the optimum A/F ratio should be for that particular moment.


Speed/density EFI systems do not have an airflow sensor. Airflow is estimated using engine RPM, inlet air temperature, throttle position and engine load (via the MAP sensor). A speed/density system is not as sophisticated as a mass airflow system, but it is less expensive for the auto maker because it requires no MAF sensor. A speed/density EFI system is also less sensitive to small air leaks that may occur in the tubing between the air cleaner and throttle, or in the intake manifold or vacuum hoses that connect to the manifold. Air leaks can throw off a mass airflow EFI system and cause it to run leaner than normal because "unmetered" air that sneaks into the engine on the down wind side of the airflow sensor is not factored into the A/F calculations. FEEDBACK FUEL CONTROL Electronic EFI systems also use one or more Oxygen (O2) sensors or Air/Fuel (A/F) Sensors to constantly fine tune the A/F mixture. Both types of sensors monitor unburned oxygen in the exhaust. The computer uses this information to determine if the A/F mixture is running rich (too much fuel, not enough air) or lean (too much air, not enough fuel). This creates a feedback loop that allows the EFI system to fine tune the A/F mixture beyond what the other sensor inputs are telling the computer.

The upstream oxygen sensor tells the PCM if the engine is running rich or lean so the PCM can readjust the A/F ratio. The downstream O2 sensor is used primarily to monitor catalyst efficiency. On some applications, input from the downstream O2 sensor is also used for additional long term fuel trim adjustments. WHY THE A/F RATIO IS ALWAYS CHANGING The A/F ratio is not constant but is constantly readjusting according to changing operating conditions. For example, a significantly richer A/F ratio is required following a cold start so the engine will idle smoothly without stalling. A richer mixture is also needed when the engine is under load for more power, and when the throttle suddenly snaps open to prevent hesitation. Sometimes the A/F ratio can be made leaner than usual to improve fuel economy, such as when the vehicle is cruising under light load and throttle. The fuel mixture can also be leaned out or cut off entirely during deceleration to conserve additional fuel. It all depends on the operating strategies programmed into the computer. Reprogramming the computer is sometimes necessary to correct an emissions or driveability problem.

HOW THE AIR/FUEL RATIO IS CHANGED

The A/F ratio is adjusted by varying the pulse duration, duty-cycle or on-time of the fuel injectors. Increasing the on time of each injector pulse sprays more fuel into the engine and richens the A/F mixture. Decreasing the on time of each injector pulse leans the A/F mixture and reduces the volume of fuel sprayed into the engine. The speed of the computer determines how quickly such changes can be made. Many late model EFI systems are so fast they can readjust the A/F mixture between individual cylinder firings!

Basically, the engine computer does the opposite of what the oxygen sensors are telling it. When the O2 sensors read lean, the computer richens the A/F ratio and increases injector duration to richen the fuel mixture. When the O2 sensors read rich, the computer shorten the injector pulses to lean the fuel mixture.

Fuel trim values can be read on a scan tool.

The STFT readings shown here are abnormally high
because the engine is off. As soon as the engine starts,
the actual values will start to display.

MONITORING FUEL TRIM

The computer monitors the constantly changing A/F ratio by tracking "short term fuel trim" (STFT) and "long term fuel trim" (LTFT). You can view the fuel trim values on a scan tool to get a good idea of what kind of corrections are being made, and whether or not these corrections indicate a possible problem.

STFT is what's happening right now. STFT is constantly jumping around as the computer adjusts and readjusts the A/F ratio.

LTFT is how the fuel mixture is trending over time, and is the best indicator to tell you if the engine is running too rich or too lean and is possibly experiencing an emissions problem.

POSITIVE (+) fuel trim numbers mean the EFI system is adding fuel to compensate for a LEAN A/F mixture.

NEGATIVE (-) fuel trim numbers mean the EFI system is subtracting fuel to compensate for a RICH A/F mixture.

The changes in fuel trim are based on what the oxygen sensors are telling the engine computer. Consequently, fuel trim values and adjustments are only as accurate as the oxygen sensor inputs.

Any number of things can create inaccurate oxygen sensor readings. A misfiring spark plug, a leaky exhaust valve, or an air leak in the exhaust manifold gasket can allow false air into the exhaust. This will cause the O2 sensor to read leaner than it should, which in turn will cause the computer to richen the fuel mixture unnecessarily and increase the fuel trim numbers. This can hurt both fuel economy and emissions.

Conversely, if an old oxygen sensor is sluggish, contaminated, unresponsive or dead, it may underreport exhaust oxygen levels. This will mislead the computer into thinking the mixture is richer than it really is. The computer will lean out the mixture unnecessarily, causing the fuel trim numbers to go negative. An overly lean A/F mixture will increase NOx emissions, increase the risk of detonation (spark knock), and increase the risk of lean misfire -- which will cause a drop in performance and fuel economy, as well as a big jump in hydrocarbon emissions.

Ideally, STFT and LTFT should both be within a few percentage points of zero when the engine is idling or being held at a steady RPM and load. Good LTFT readings should be as close to zero as possible, though they often range from 5 to 8 percent depending on the condition of the engine. If LTFT numbers are more than 10, it usually indicates a problem that needs to be diagnosed. LTFT values that go as high as 20 or higher will usually set a P0171 or P0174 lean code and turn on the Check Engine light.

High positive LTFT fuel trim numbers due to a lean A/F mixture may be the result of dirty fuel injectors, a dirty mass airflow sensor, vacuum leaks at the intake manifold or in vacuum hoses, a weak fuel pump, or a faulty fuel pressure regulator that cannot maintain adequate pressure to the injectors.

Electronics cleaner can be used to clean a dirty MAF sensor, while fuel cleaner or an off-car injector cleaning machine can be used to clean dirty injectors. Vacuum leaks can often be found by lightly pressurizing the intake manifold (no more than 3 or 4 PSI) and spraying soapy water on suspected leak points. Bubbles anywhere indicate a leak. Mineral oil vapor can also be fed into the intake manifold under light pressure using a smoke machine to search for small leaks.

Excessive negative LTFT fuel trim numbers caused by a rich A/F mixture may be the result of oxygen sensors that need to be replaced, false air getting into the exhaust, compression leaks, ignition misfire (check the spark plugs and coils), one or more leaky fuel injectors, or excessive fuel pressure due to a faulty fuel pressure regulator.

Fuel System Diagnostics

With today's computerized engine controls and electronic fuel injection, it's hard to separate the fuel system from other systems when it comes to driveability and emissions diagnostics. Symptoms such as hard starting, stalling, hesitation, loss of power, poor fuel economy, rough idle, misfiring and elevated emissions can be caused by any number of things. So your job is to zero in on the most likely causes using the quickest and most effective procedures at your disposal. Sometimes this means ruling out other possibilities first, such as ignition or compression problems. Once these have been eliminated, you can focus on the fuel system and try to isolate the problem using the appropriate tests.

Before we go any further, let's define which parts we are talking about. The fuel system includes everything from the fuel filler cap, fuel tank, fuel pump, pump relay, fuel lines and filter to the fuel injectors, pressure regulator, fuel rail and throttle body. But our list may also include other parts and systems that influence or control the operation of the fuel system such as the PCM, oxygen sensor, coolant sensor, MAP sensor, throttle position sensor and airflow sensor (if one is used). There is also the idle speed control system, and the evaporative emissions system that captures and manages fuel vapors. We also have to include the fuel itself because "bad gas" that has been contaminated with water or higher-than-normal amounts of alcohol additives is still an often-overlooked cause of common driveability problems.

Basic Causes of Fuel Injection Problems Fuel-related driveability and emission problems can be lumped into one of three basic categories: Code but no obvious driveability or emissions problem; Code and a driveability or emissions problem; or No code but a driveability or emissions problem. The first two are usually easier to diagnose because you at least have a starting point (the fault code). The last one (no code) is the most challenging because you have only the symptom(s). If the Check Engine light is on, you know the computer has detected something wrong and has logged one or more diagnostic trouble codes that correspond to the fault(s). When you hook up your scan tool, you know you are going to find some kind of information that should get you headed in the right diagnostic direction. Without getting too philosophical, we all know that some codes are more helpful than others. It depends on the code, the conditions that are required to set it and the diagnostic procedures that follow it. A code that indicates a particular sensor is reading out of range is usually a good indication that there is a problem in the sensor circuit. But if that code is accompanied by other codes, it often indicates an operating condition that is throwing the sensors off. When there is more than one code, it sometimes makes matters worse because you may not be sure where to start. Should you go in numerical sequence, or should you step back and try to figure out what kind of condition would cause multiple codes to be set?

What Might Be Causing Your Fuel Injection Problem? Let's assume you have a late-model OBD II car with a four-cylinder engine. The Check Engine light is on and the engine idles a little rough and has some hesitation. You hook up your scan tool and find a P0131 code that indicates the O2 sensor is reading low (lean) and a P0300 code for a random misfire. Is the O2 sensor bad or is it something else? The problem might be a faulty O2 sensor, but more likely causes include a vacuum leak, low fuel pressure or dirty injectors that are leaning out the fuel mixture, causing the engine to run poorly and misfire. It might even be bad gas. What you do next will depend on the type of diagnostic equipment you have and what you think is the most likely cause. You might begin your investigation by inspecting the intake manifold and vacuum hoses for obvious leaks. Finding none, you might hook up a vacuum gauge to see if intake vacuum readings are steady at idle and within normal range. A low reading could indicate a vacuum leak somewhere or possibly a leaking EGR valve. Further investigation would be needed to determine the cause if the readings are abnormal. Next, you might use your scan tool, a DVOM or scope to look at O2 sensor performance. If the O2 sensor responds normally when you temporarily enrichen the fuel mixture by feeding some propane into the engine, or lean the mixture by disconnecting a vacuum line, you can probably assume the O2 sensor is OK. And if the engine control system is in closed loop (check loop status) and changes the injector dwell in response to changes in the O2 sensor reading, you can assume the feedback fuel control loop is doing its job, too. Next, you might connect a pressure gauge to the fuel rail and check fuel pressure and the operation of the fuel pressure regulator. Does the pressure change when you disconnect the regulator vacuum line or pinch off the return hose? Low pressure readings would lead you to suspect a weak pump, while no change in the pressure readings when playing with the regulator would point toward a faulty regulator. If fuel pressure is normal, you might look at the injector patterns on your scope. Does the dwell change when you goose the throttle or make the mixture go rich or lean? You might also use your scope to look at the engine's secondary ignition pattern. When one or more (but not all) of the spark firing lines slope up to the right and firing time shortens, it indicates a lean fuel condition in the affected cylinders. A rich mixture is more conductive than a lean one, and requires less voltage to sustain a spark. On a fuel-injected engine, this could be a symptom of a dirty, clogged or inoperative fuel injector. The premature snuffing out of the spark can also be caused by poor cylinder breathing due to a rounded cam lobe or a leaky exhaust valve. A lean fuel mixture will also cause the firing KV to be higher than normal. If the fuel mixture is running rich because the O2 sensor is dead or the feedback control system is stuck in open loop, the firing line portion of the secondary ignition pattern will show hairlike extensions hanging from the spark line. This is caused by the increased conductivity of the rich mixture. The KV firing voltages will also be lower than normal with a longer-than-normal spark duration.

Another alternative would be to hook up an exhaust emissions analyzer to look at carbon monoxide (CO) and hydrocarbons (HC). Higher-than-normal CO would tell you the mixture is running rich, while elevated HC would tell you there is a misfire or compression problem (lean misfire, ignition misfire or a burned exhaust valve). If you use the exhaust analyzer in conjunction with a power balance test, you can see if the HC readings change when each injector is shorted out. No change in HC would indicate a bad injector. If the change in HC is noticeably less on one cylinder than the others, it would tell you that injector is probably restricted and needs to be cleaned or replaced.

If you suspect dirty injectors, your next step might be to clean the injectors on the car. Or, if you have an off-car injector cleaner, you might pull the injectors, flow test them on the bench and clean them to see if normal performance can be restored (if not then you will have to replace the injectors).

According to some experts, a difference of more than 7 to 10% in flow rates between individual injectors on a multiport system is enough to cause noticeable driveability problems. Some cylinders will run too rich while others will run too lean. Ideally, all the injectors should flow within 3 to 5% of one another. If they don't, the computer will compensate for the leanest injector, forcing all the others to run rich.

When There Is No Fault Code No-code driveability and emission problems are the ones everybody hates, especially when the symptoms are intermittent. In such situations, you have to go look at the symptom(s) and make some educated guesses as to what to look at first. As before, how you approach a no-code problem will depend on what you have in your diagnostic arsenal. If you have an emissions analyzer, you might look at exhaust gases first. If you have a scope, you might examine sensor, injector, ignition or even the fuel pump waveforms. Since most technicians today own some type of scan tool, checking system data is as good a place as any to start, even when there is no code. One of the first things you should check is loop status. The fuel system cannot deliver the proper fuel mixture if is stuck in open loop. If the engine fails to go into closed loop after it has warmed up or been driven, it may have a faulty coolant sensor or an open thermostat. The next logical step would be to look at the coolant sensor's output to see if it is reading normally or if its resistance changes as the engine warms up. No change in resistance or a reading that is out of range would tell you the coolant sensor is bad. To check the thermostat, you could use an infrared thermometer to measure coolant temperature at the thermostat outlet after the engine has warmed up. If low, the thermostat may be open, missing or the wrong temperature rating for the engine. You can also look at short-term and long-term fuel trim to see if the engine is running rich or lean. If the system is going into closed loop, look at the TPS and MAP inputs to make sure they are changing when the throttle position changes. For more information about feedback fuel control, see What Is Fuel Trim? Expert Fuel System Troubleshooting At the last International Automotive Technicians Network (iATN) convention in Dearborn, MI, a roomful of top technicians from all over the country was challenged to correctly diagnose several real-world driveability problems. They were given a description of some vehicles and their symptoms, then allowed to view actual test data that was taken from the vehicles using a variety of different diagnostic tests and equipment. The challenge was to figure out which diagnostic tests and equipment would provide the most useful information, and to then analyze this information to come up with a diagnosis. It proved to be a difficult task because opinions differed as to what data should be looked at first, and what the data actually meant. Some people resorted to making wild guesses in an attempt to solve the problems.

One example that stumped a lot of technicians was a car that would start and run fine when cold, but had a hot-start problem. It would crank but not start. It had fuel, spark and compression. Fuel pressure and vacuum readings were normal. Coil resistance, spark plugs and wires were all within specifications.

The hot-start problem turned out to be a bad coolant sensor. The sensor was preventing the fuel system from going into closed loop, which created a rich mixture that flooded the engine when a hot start was attempted.

The diagnostic approach for solving a no-start condition that evolved from this was to:

1. Check for spark to rule out ignition problems.
2. Check for fuel to rule out an empty tank, dead fuel pump or plugged fuel line.
3. Check compression to rule out a broken cam, timing chain or OHC timing belt.

If all of the above are OK, remove and examine a spark plug. If the plugs are wet, it would tell you the engine is flooded due to too much fuel. The underlying cause would be a defective coolant sensor or leaky injector(s).

Another approach when confronted with a no-start condition is to check cranking vacuum. It should usually be at least 3". If lower than this, there is a compression problem. Also, look at the amount of HC in the exhaust. If you see at least 7,500 ppm of HC, there should be enough fuel to start. If the HC reading is below 7,500 ppm, there is not enough fuel.

On many vehicles, a no-start can occur if the oil pressure switch is bad. The oil pressure switch is wired into the fuel pump relay circuit to cut voltage if oil pressure is lost. This is to prevent fuel from spraying out of a ruptured fuel line in the event of an accident. Some vehicles also have an inertia crash switch hidden somewhere in the bodywork (look in the trunk, under the back seat or inside the rear seat side panels) to cut off fuel in case of an accident. An inertia switch can be reset by pressing its reset button.

Hard starting on some older fuel injected systems can be caused by a faulty cold start injector. A timed relay energizes the injector when the engine is cranked to provide extra fuel. Most problems here can be traced to electrical faults in the control relay or wiring. Use a test light to check the cold-start injector when the engine is cranked. No voltage means the relay (or its fuse) is bad.

Leaky Fuel Injectors

Wear in the injector orifice and/or accumulated deposits can sometimes prevent the pintle valve inside an injector from seating, allowing fuel to dribble out the nozzle. The extra fuel causes a rich fuel condition, which can foul spark plugs, increase emissions and cause a rough idle. A carbon-fouled spark plug in one cylinder of a multi-port EFI engine usually indicates a leaky injector. If cleaning fails to eliminate the leak (which it can if dirt or varnish are responsible), replacement will be necessary.

Idle Problems

Idle problems can usually be traced to an air or vacuum leak, or in some cases, a faulty idle air bypass control motor or plugged idle bypass circuit. Leaks allow "unmetered" air into the engine and lean out the fuel mixture. The computer compensates by richening the mixture and closing off the idle bypass circuit. One of the symptoms of a vacuum leak, therefore, is a bypass motor run completely shut.

You can use your scan tool to check for a vacuum leak by looking at the fuel trim values. If the engine has a vacuum leak, the fuel trim values will usually be higher than normal (positive) when the engine is idling, but will drop back more towards normal when engine speed is increased. This is because a vacuum leak has less affect on the fuel mixture at higher engine speeds than at idle.

Plugged Fuel Filter

A plugged filter can restrict the flow of fuel and starve the engine, causing a loss of power at high speed, a lean fuel mixture or a stalling/no-start condition if the blockage is severe. The easiest way to check the filter is to remove it and attempt to blow air through it.

If the filter is plugged with rust or sediment, it is probably a good idea to drain and clean the fuel tank to prevent a repeat failure. If the tank is badly corroded inside, replacing the fuel tank would be recommended.

A restricted filter sock on the fuel pump pickup can cause similar symptoms, too. So, if the filter appears to be OK or replacing it fails to solve the problem, it may be necessary to drop the tank, remove the pump and inspect, clean or replace the filter sock.

Diagnose Electronic Fuel Injection

Electronic fuel injection is a great means of delivering fuel to an engine. With multiport systems, each cylinder receives its own dose of fuel, and with sequential controls, the air/fuel ratio for each cylinder can be quickly changed to keep in step with changes in engine load. EFI also improves cold starting, reduces emissions, improves fuel economy and performance. But sometimes things go amiss. Fuel injection problems encompass everything from hard starting, stalling and misfiring to hesitation, surging and no-starts. Dirty injectors, for example, will restrict the amount of fuel that is sprayed into the engine with every pulse of the injector resulting in a leaner-than-normal fuel mixture. This, in turn, can cause lean misfire, hesitation, poor performance and an increase in hydrocarbon (HC) emissions. Because EFI is part of the powertrain control module's feedback control loop, 

problems with the coolant sensor or oxygen sensor also can affect fuel delivery. A coolant sensor that always reads cold will prevent the engine from going into closed loop resulting in a rich fuel mixture and poor fuel economy. A dead oxygen sensor can have the same effect. So too, can a contaminated or sluggish O2 sensor. The PCM also relies on inputs from the throttle position sensor, airflow sensor (if one is used), manifold absolute pressure (MAP) sensor and intake air temperature sensors to adjust the fuel mixture. Most sequential injection systems use the signal from the camshaft or crankshaft position sensor to trigger and sync the injector pulses. Problems in any of these sensor circuits also can affect fuel delivery. Most PCMs have an internal driver circuit for grounding (or in some cases energizing) the injectors. Problems here can disable one or more injectors depending on how the system is wired. In some cases, a shorted injector will kill the driver circuit in the PCM. And don't forget the power relay that supplies voltage to the injectors. If this relay dies, it will disable all of the injectors. There's also the components in the fuel system itself: the fuel pump, pump relay, fuel filter, fuel lines, pressure regulator and injectors. Problems with any of these components may prevent fuel from reaching the engine or reaching it at the correct pressure. fuel pressure regulator A leaky pressure regulator can cause a no-start. The point here is a lot of things can affect the operation of the fuel delivery system. The challenge is to figure out what is causing the problem without wasting a lot of time chasing dead ends. We don't have the space to explore every possibility, so we'll focus on the main components in the fuel system itself. Cranks But Won't Start Where do you begin your diagnosis if you have an engine that cranks but won't start? One of the first things you should do is check for spark. Got spark? What about compression? If the engine has a belt-driven cam, make sure the belt has not failed. Also, check for any loose hoses that might be creating a huge vacuum leak. If ignition and compression are both OK, that leaves fuel as the obvious culprit. Now the question is, what is wrong with the fuel delivery system? The most likely causes are: 1. A dead fuel pump (could be the pump, pump relay or wiring circuit); 2. A plugged fuel filter; 3. Low fuel pressure (weak pump or restricted line); or 4. No pulse signal to injectors (bad injector relay or PCM driver circuit). One of the first things to check is the fuel pump. Does the pump run when the engine is cranking? The pump should make a little noise. No noise would tell you the pump is not spinning. fuel pump fuse On most vehicles the pump is energized by the PCM via a relay. The pump circuit also may be wired though an oil pressure switch and/or an inertia safety switch that kills the pump in case of an accident. Refer to the wiring diagram to find out what is involved before jumping to any conclusions. Other electrical problems that can affect the pump include low voltage in the pump's power supply circuit or high resistance in the pump's ground connection. Either may prevent the pump from running or spinning fast enough to generate normal fuel pressure. test fuel injection pump draw Fuel Pressure Checks Depending on the application, the fuel system may require anywhere from 30 to 80 psi of fuel pressure to start and run. Pressure specifications will vary according to the type of fuel injection system on the engine as well as the performance, fuel economy and emission requirements of that particular model year vehicle. There are no rules of thumb. Every application is different, so always look up the pressure specs when troubleshooting fuel-related performance problems. When there is too much fuel pressure, the engine runs rich. This causes an increase in fuel consumption and carbon monoxide (CO) emissions. An engine that is running really rich also may experience a rough idle, surging and possibly even carbon-fouled spark plugs. When there is not enough fuel pressure, the engine may not start. Or if it does, it may idle roughly and run poorly. Low fuel pressure creates a lean fuel condition that can cause lean misfire, hesitation, rough idle, hesitation and misfire on acceleration. To check fuel pressure, you need a gauge and a place to attach it. There are a number of different checks that can be made, including static or rest pressure (key on, engine off), residual fuel pressure, running pressure, maximum or "dead head" pressure and volume of fuel delivered. The fuel pressure regulator also should be tested, and a fuel pressure drop test performed to check for dirty fuel injectors. Different vehicle manufacturers recommend different test procedures. On many European EFI systems, the OEMs recommend using a static pressure test with the engine and ignition off. This is done by bypassing the fuel pump relay and energizing the pump directly. Most domestic and Asian vehicle manufacturers, on the other hand, provide a test fitting on the fuel rail so pressure can be checked with the engine running. If you are working on a vehicle that does not have a pressure test fitting, you will have to tee a pressure gauge into the fuel line just ahead of the injector fuel rail. Caution: Before hooking up your pressure gauge, relieve all pressure in the fuel system. fuel rail test port Most fuel injection systems have a schrader valve like this on the fuel rail to check fuel pressure. Static Fuel Pressure Test With the key on, engine off (or with the fuel pump energized), fuel pressure should come up quickly and hold steady at a fixed value. Compare the pressure reading to specifications. If you get no pressure reading, check for voltage at the pump. If there is voltage but the pump is not running, you have found the problem: a bad fuel pump. If you do get a pressure reading but the reading is lower than normal, the cause may be a weak pump, a blockage in the fuel line, filter or tank inlet sock, or a faulty pressure regulator. Also, low voltage at the pump may prevent it from spinning fast enough to build up normal pressure. Check the voltage at the pump. If OK, check the fuel filter and lines for obstructions and the operation of the fuel pressure regulator before you condemn the pump. Residual Fuel Pressure Test When the pump is turned off or stops running, the system should hold residual pressure for several minutes (look up the specs to see how much pressure drop is allowed over a given period of time). If pressure drops quickly, the vehicle may have a leaky fuel line, a leaky fuel pump check valve, a leaky fuel pressure regulator or one or more leaky fuel injectors. Low residual fuel pressure can cause hard starting and vapor lock during hot weather. Running Fuel Pressure Test With the engine idling, compare the gauge reading to specifications. Fuel pressure should be within the acceptable range given by the vehicle manufacturer. If low, the problem may be a weak pump, low voltage to the pump, a clogged fuel filter, line or inlet sock inside the fuel tank, a bad pressure regulator, or nearly empty fuel tank. Dead Head Pressure This checks the maximum output pressure of the fuel pump. With the return line pinched shut, the pump should produce two times its normal operating pressure at idle. If the pressure rating does not go up with the return line blocked, the pump may not be able to deliver enough fuel at higher engine speeds. Possible causes include a worn pump, low voltage at the pump, a plugged fuel filter or inlet sock in the tank, an obstructed fuel line or almost empty fuel tank. Fuel Volume Test A fuel pump that delivers normal pressure may still cause driveability problems if it can't deliver enough fuel volume to meet the engine's needs. A fuel volume test may therefore be the best way to evaluate the pump's condition. A fuel volume test measures the volume of fuel delivered over a specified interval. This test can be done by connecting a fuel flow gauge into the fuel supply line, or by disconnecting the fuel return line from the fuel pressure regulator and connecting a hose from the regulator to a large container. Caution: Make sure there are no open sparks or flames nearby while doing this test! With the engine off, energize the pump and measure the volume of fuel delivered during the specified interval of time. As a rule, a good pump should deliver about 3/4 to one quart of fuel in 30 seconds. Causes of low fuel volume delivered include a worn fuel pump, a plugged fuel filter or inlet sock in the tank, obstructed fuel line or nearly empty tank. Don't forget that low voltage at the pump can also prevent it from running fast enough to generate adequate fuel flow. The pump's supply voltage should be within half a volt of normal system voltage. If it is low, check the wiring connectors, relay and ground. Fuel Pressure Regulator Test This test checks the operation of the fuel pressure regulator to make sure it changes line pressure in response to changes in engine vacuum. This is necessary to maintain the proper operating pressure behind the injectors and to compensate for changes in engine load. With the engine running, disconnect the vacuum hose from the pressure regulator. As a rule, fuel system pressure should increase 8 to 10 psi with the line disconnected. No change would indicate a faulty pressure regulator, or a leaky or plugged vacuum line. If the diaphragm inside the regulator is leaking, engine vacuum will suck raw fuel into the intake manifold through the vacuum hose (look for fuel inside the hose). Fuel Pressure Drop Test This test measures the drop in static system fuel pressure when each injector is energized. The amount of pressure drop for each injector is then compared to see if the injectors are dirty and need to be cleaned or replaced. This test requires an "injector pulser" tool to energize the injectors. To perform this test, turn the key on or energize the fuel pump for a few seconds to build up static pressure in the fuel system. Then turn the key off, pulse one injector for the specified time and note the pressure drop for that injector. Turn the key back on to rebuild static pressure and repeat the test for each of the remaining injectors. An injector that is pulsed 100 times for five milliseconds should produce a minimum pressure drop of about 1 to 3 psi, and no more than 5 to 7 psi, depending on the application. The difference in pressure drop between all the injectors should be 2 psi or less. If you see more than 3 psi difference between the highest and lowest readings, the injectors are dirty and need to be cleaned or replaced. If you see no pressure drop when an injector is energized, the injector is clogged or defective and needs to be replaced. If the pressure gauge needle bounces, the injector is sticking. fuel pump After cleaning, repeat the test to see if cleaning did the trick. All injectors should show about the same amount of pressure drop (less than 2 psi difference) and no more than 7 psi drop at 100 pulses for 5 milliseconds. If there is no change in the readings or the drop exceeds these limits, the injector(s) need to be replaced. Scope Tests By connecting a Low Amps probe to the fuel pump's voltage supply wire, you can create a scope waveform that will reveal internal wear in the brushes and commutator that may not show up in a traditional pressure or volume test. Observing the waveform will tell you if the pump's amp draw is normal for the application or is high or low, and if the pump is operating at normal speed or is running slow. Problems such as a bad spot on a commutator or a short or open in the armature also will be obvious in the waveform. A "good" electric fuel pump waveform will generally seesaw back and forth with relative consistency and minimal variation between the highs and lows. A "bad" waveform will show large or irregular drops in the pattern, with large differences between the highs and lows. Pump Replacement If you have diagnosed a bad fuel pump and replacement is needed, be sure to inspect the inside of the fuel tank. The presence of rust or debris in the pump's pickup screen would tell you the tank needs to be cleaned or replaced. When replacing an in-tank pump, always disconnect the battery to prevent any unwanted sparks. Then drain the tank before removing the tank straps and opening the pump's retaining collar. When installing the new pump, always replace the in-tank pickup screen and use a new O-ring for the sealing collar.

Fuel Trim Diagnostics To maintain the correct air/fuel mixture, the PCM adjusts Short Term Fuel Trim (STFT) and Long Term Fuel Trim (LTFT). Both of these values can be read on a scan tool that can display OBD II system data, and may be used to diagnose fuel-related problems. Normal range for both STFT and LTFT is typically plus or minus 8. If the numbers are +10 or higher for STFT and LTFT, the engine is running LEAN (not enough fuel). If the values for both STFT and LTFT are both minus 10 or more, the engine is running RICH (too much fuel). For more information on this subject, see What Is Fuel Trim? Vacuum leaks can create a lean-running engine that requires extra fuel to balance the fuel mixture. If you rev the engine to 1500 to 2000 rpm and hold it for a minute or so, and the STFT value drops back down to a more normal reading, it confirms the engine has a vacuum leak at idle. If the STFT value does not change much, the lean fuel condition is more likely a fuel delivery problem (weak fuel pump, restricted fuel filter, dirty fuel injectors or a leaky fuel pressure regulator) than a vacuum leak. For more information about using fuel trim to diagnose a lean fuel condition, read this article on Fuel Trim by Wells Manufacturing (PDF file, requires Adobe Acrobat to read).

Fuel injection problems encompass everything from hard starting, stalling and misfiring to hesitation, surging and no-starts. Dirty injectors, for example, will restrict the amount of fuel that is sprayed into the engine with every pulse of the injector resulting in a leaner-than-normal fuel mixture. This, in turn, can cause lean misfire, hesitation, poor performance and an increase in hydrocarbon (HC) emissions.

Because EFI is part of the powertrain control module's feedback control loop, problems with the coolant sensor or oxygen sensor also can affect fuel delivery. A coolant sensor that always reads cold will prevent the engine from going into closed loop resulting in a rich fuel mixture and poor fuel economy. A dead oxygen sensor can have the same effect. So too, can a contaminated or sluggish O2 sensor. The PCM also relies on inputs from the throttle position sensor, airflow sensor (if one is used), manifold absolute pressure (MAP) sensor and intake air temperature sensors to adjust the fuel mixture. Most sequential injection systems use the signal from the camshaft or crankshaft position sensor to trigger and sync the injector pulses. Problems in any of these sensor circuits also can affect fuel delivery. Most PCMs have an internal driver circuit for grounding (or in some cases energizing) the injectors. Problems here can disable one or more injectors depending on how the system is wired. In some cases, a shorted injector will kill the driver circuit in the PCM. And don't forget the power relay that supplies voltage to the injectors. If this relay dies, it will disable all of the injectors. There's also the components in the fuel system itself: the fuel pump, pump relay, fuel filter, fuel lines, pressure regulator and injectors. Problems with any of these components may prevent fuel from reaching the engine or reaching it at the correct pressure.

A leaky pressure regulator can cause a no-start.

The point here is a lot of things can affect the operation of the fuel delivery system. The challenge is to figure out what is causing the problem without wasting a lot of time chasing dead ends. We don't have the space to explore every possibility, so we'll focus on the main components in the fuel system itself. Cranks But Won't Start Where do you begin your diagnosis if you have an engine that cranks but won't start? One of the first things you should do is check for spark. Got spark? What about compression? If the engine has a belt-driven cam, make sure the belt has not failed. Also, check for any loose hoses that might be creating a huge vacuum leak. If ignition and compression are both OK, that leaves fuel as the obvious culprit. Now the question is, what is wrong with the fuel delivery system? The most likely causes are: 1. A dead fuel pump (could be the pump, pump relay or wiring circuit); 2. A plugged fuel filter; 3. Low fuel pressure (weak pump or restricted line); or 4. No pulse signal to injectors (bad injector relay or PCM driver circuit). One of the first things to check is the fuel pump. Does the pump run when the engine is cranking? The pump should make a little noise. No noise would tell you the pump is not spinning.

On most vehicles the pump is energized by the PCM via a relay. The pump circuit also may be wired though an oil pressure switch and/or an inertia safety switch that kills the pump in case of an accident. Refer to the wiring diagram to find out what is involved before jumping to any conclusions. Other electrical problems that can affect the pump include low voltage in the pump's power supply circuit or high resistance in the pump's ground connection. Either may prevent the pump from running or spinning fast enough to generate normal fuel pressure.

Fuel Pressure Checks Depending on the application, the fuel system may require anywhere from 30 to 80 psi of fuel pressure to start and run. Pressure specifications will vary according to the type of fuel injection system on the engine as well as the performance, fuel economy and emission requirements of that particular model year vehicle. There are no rules of thumb. Every application is different, so always look up the pressure specs when troubleshooting fuel-related performance problems. When there is too much fuel pressure, the engine runs rich. This causes an increase in fuel consumption and carbon monoxide (CO) emissions. An engine that is running really rich also may experience a rough idle, surging and possibly even carbon-fouled spark plugs. When there is not enough fuel pressure, the engine may not start. Or if it does, it may idle roughly and run poorly. Low fuel pressure creates a lean fuel condition that can cause lean misfire, hesitation, rough idle, hesitation and misfire on acceleration. To check fuel pressure, you need a gauge and a place to attach it. There are a number of different checks that can be made, including static or rest pressure (key on, engine off), residual fuel pressure, running pressure, maximum or "dead head" pressure and volume of fuel delivered. The fuel pressure regulator also should be tested, and a fuel pressure drop test performed to check for dirty fuel injectors. Different vehicle manufacturers recommend different test procedures. On many European EFI systems, the OEMs recommend using a static pressure test with the engine and ignition off. This is done by bypassing the fuel pump relay and energizing the pump directly. Most domestic and Asian vehicle manufacturers, on the other hand, provide a test fitting on the fuel rail so pressure can be checked with the engine running. If you are working on a vehicle that does not have a pressure test fitting, you will have to tee a pressure gauge into the fuel line just ahead of the injector fuel rail. Caution: Before hooking up your pressure gauge, relieve all pressure in the fuel system.

Most fuel injection systems have a schrader valve like this
on the fuel rail to check fuel pressure.

Static Fuel Pressure TestWith the key on, engine off (or with the fuel pump energized), fuel pressure should come up quickly and hold steady at a fixed value. Compare the pressure reading to specifications. If you get no pressure reading, check for voltage at the pump. If there is voltage but the pump is not running, you have found the problem: a bad fuel pump.

If you do get a pressure reading but the reading is lower than normal, the cause may be a weak pump, a blockage in the fuel line, filter or tank inlet sock, or a faulty pressure regulator. Also, low voltage at the pump may prevent it from spinning fast enough to build up normal pressure. Check the voltage at the pump. If OK, check the fuel filter and lines for obstructions and the operation of the fuel pressure regulator before you condemn the pump.

Residual Fuel Pressure TestWhen the pump is turned off or stops running, the system should hold residual pressure for several minutes (look up the specs to see how much pressure drop is allowed over a given period of time). If pressure drops quickly, the vehicle may have a leaky fuel line, a leaky fuel pump check valve, a leaky fuel pressure regulator or one or more leaky fuel injectors. Low residual fuel pressure can cause hard starting and vapor lock during hot weather.

Running Fuel Pressure TestWith the engine idling, compare the gauge reading to specifications. Fuel pressure should be within the acceptable range given by the vehicle manufacturer. If low, the problem may be a weak pump, low voltage to the pump, a clogged fuel filter, line or inlet sock inside the fuel tank, a bad pressure regulator, or nearly empty fuel tank.

Dead Head PressureThis checks the maximum output pressure of the fuel pump. With the return line pinched shut, the pump should produce two times its normal operating pressure at idle. If the pressure rating does not go up with the return line blocked, the pump may not be able to deliver enough fuel at higher engine speeds. Possible causes include a worn pump, low voltage at the pump, a plugged fuel filter or inlet sock in the tank, an obstructed fuel line or almost empty fuel tank.

Fuel Volume TestA fuel pump that delivers normal pressure may still cause driveability problems if it can't deliver enough fuel volume to meet the engine's needs. A fuel volume test may therefore be the best way to evaluate the pump's condition.

A fuel volume test measures the volume of fuel delivered over a specified interval. This test can be done by connecting a fuel flow gauge into the fuel supply line, or by disconnecting the fuel return line from the fuel pressure regulator and connecting a hose from the regulator to a large container. Caution: Make sure there are no open sparks or flames nearby while doing this test!

With the engine off, energize the pump and measure the volume of fuel delivered during the specified interval of time. As a rule, a good pump should deliver about 3/4 to one quart of fuel in 30 seconds.

Causes of low fuel volume delivered include a worn fuel pump, a plugged fuel filter or inlet sock in the tank, obstructed fuel line or nearly empty tank. Don't forget that low voltage at the pump can also prevent it from running fast enough to generate adequate fuel flow. The pump's supply voltage should be within half a volt of normal system voltage. If it is low, check the wiring connectors, relay and ground.

Fuel Pressure Regulator TestThis test checks the operation of the fuel pressure regulator to make sure it changes line pressure in response to changes in engine vacuum. This is necessary to maintain the proper operating pressure behind the injectors and to compensate for changes in engine load.

With the engine running, disconnect the vacuum hose from the pressure regulator. As a rule, fuel system pressure should increase 8 to 10 psi with the line disconnected. No change would indicate a faulty pressure regulator, or a leaky or plugged vacuum line.

If the diaphragm inside the regulator is leaking, engine vacuum will suck raw fuel into the intake manifold through the vacuum hose (look for fuel inside the hose).

Fuel Pressure Drop TestThis test measures the drop in static system fuel pressure when each injector is energized. The amount of pressure drop for each injector is then compared to see if the injectors are dirty and need to be cleaned or replaced. This test requires an "injector pulser" tool to energize the injectors.

To perform this test, turn the key on or energize the fuel pump for a few seconds to build up static pressure in the fuel system. Then turn the key off, pulse one injector for the specified time and note the pressure drop for that injector. Turn the key back on to rebuild static pressure and repeat the test for each of the remaining injectors.

An injector that is pulsed 100 times for five milliseconds should produce a minimum pressure drop of about 1 to 3 psi, and no more than 5 to 7 psi, depending on the application.

The difference in pressure drop between all the injectors should be 2 psi or less. If you see more than 3 psi difference between the highest and lowest readings, the injectors are dirty and need to be cleaned or replaced.

If you see no pressure drop when an injector is energized, the injector is clogged or defective and needs to be replaced. If the pressure gauge needle bounces, the injector is sticking.