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 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.
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.
8 сен 2024
