"Fuel trim" in the ECU is the ECU's method of tuning individual cars.
In theory, the ECU reads the amount of air entering the engine via the mass airflow sensor (MAS) in the air intake. It will then add the proper amount of fuel through the injectors, ensuring a good burn, low emissions, etc.
However, while that works in theory, it rarely does in practice. MAS sensors vary, even when calibrated, even if only due to age. Injectors become clogged, fuel and turbo pressures can vary, parts wear or are replaced.
The way to compensate for the individual car-to-car variations is through fuel trim. By checking the oxygen sensor, the ECU can "check" if the amount of fuel it added was too high or too low. It will then add or subtract fuel in small increments depending on what it "sees". It "remembers" these adjustments and averages them over a period of time, resulting in a "trim" factor. The fuel trim is constantly applied and adjusted.
There are three fuel trims on DSMs: low, medium and high. These refer to the mass of air entering the engine, and only roughly correspond to RPM ranges. Low applies mostly to idle (750-1000 RPM), high in the 3000-4000 region, and middle in between.
Users of Super-AFC and similar units usually try to adjust their AFCs so the fuel trims are near 100%. This is mostly to provide a baseline of settings for future tuning - it means the AFC settings are "fooling" the ECU into "thinking" the car is still stock (or near stock). It also allows the ECU a lot of range to adjust the fuel trims as necessary.
Fuel trims have no effect at open loop (high RPM or hard accelleration). In this situation the ECU is ignoring the oxygen sensor and there is nothing to trim. Datalogger owners will see their fuel trim "stick" at 100% in open-loop situations.
Those interested in the details of DSM datalogging may find the Pocketlogger FAQ and Technomotive's Datalogger 101guide.
For ECMLink Users, view Tuning with ECMLink article
Last Updated:
2016-08-08 01:32
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