Adaptive Learning / Fuel Trim
Fuel trim is a two part adaptive strategy that adjusts fuel injectors pulse width (open time) to adapt to any defects or changes in the system. The two parts of a computers fuel trim adaptive strategy are short term fuel trim STFT(multiplicative) and long term fuel trim LTFT (additive). Short term fuel trim occurs in short temporary adjustments that happen immediately. Long term fuel trim is accumulated over time and stored in memory. It is often the result of several STFT adjustments that exceeded 25% increase or decrease over time. The STFT adaptive strategies use drive maps written after extensive tests performed by engineers using dynamometers.
The PCM tries to maintain a long term air/fuel ratio of 14.7:1. If it sees that the long term ratio is not maintained at stoichiometry (14.7:1), then it can add or subtract from the injectors base pulse width "open time" to compensate. Injector on time is the result of sensor input and learned adaptive strategies implemented by the PCM. If the scanners LTFT indicate -.02 then the PCM is reducing on time. If scanner LTFT indicate +.02 then the PCM is increasing injector on time. A "minus" is subtracting fuel and a "plus" is adding fuel.
The illustration above shows the different sensors used to calculate fuel injector pulse width. The question indicates the importance of an oxygen sensors input in calculating injector pulse width. The PCM's base fuel control is based on feedback from the oxygen sensor. The PCM makes minor adjustments to the fuel ratio and checks the results with the O2 sensors input. If the oxygen sensor were stuck frozen with an erroneous and constant output of 800Mv, the base injector pulse width and air/fuel ratio would be altered.
