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.
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.
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 computers memory so the next time the vehicle is driven it can pick up where it left off. Erasing the computers 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 vehicles 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 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 vehicles 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
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 engines 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 BeIdeally, 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
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 engines 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 BeIdeally, 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
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)
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)
