Fuel Correction Or Adaptive Memories

FUEL CORRECTION or ADAPTIVE MEMORIES

OPERATION

Short Term

Short Term Adaptive or Short Term Fuel Trim (STFT) is an immediate correction to fuel injector pulse width. During Closed Loop operation, Short Term Adaptive makes immediate adjustments to fuel delivery in direct response to the signal from the upstream O2 sensor. The PCM infers air/fuel ratio by monitoring oxygen content measured by the upstream O2 stream sensor.

This is an immediate response to the O2 sensor signal that is not switching or is consistently high or low. If the upstream oxygen sensor voltage is not consistently switching between 2.5 and 3.5 volts, the PCM knows that the base pulse width calculation needs to be modified by adjusting the injector pulse width until the correct O2 sensor voltage is achieved. The need to adjust the injector pulse width may be a result of vehicle operating conditions, engine wear, fuel quality, etc. The maximum range of authority for Short Term Adaptive is ±33%. Short Term Adaptive values are not stores when the ignition is off.

Long Term

The main function of Long Term Adaptive is to make fuel corrections that permit Short Term Adaptive to hover around zero. In order to maintain correct emissions throughout all operating ranges of the engines, a cell structure based on engine rpm and load (MAP) is used.

There are 26 cells used for the NGC. Two of the cells are used only during idle, as determined by throttle position and park/neutral switch input. The other cells each represent a given off-idle manifold pressure and rpm range.

After the vehicle has reached full operating temperature, short term correction factors will update Long Term Adaptive Memory cells based on vehicle load (RPM/MAP) to allow the Short Term Adaptive value to be brought back to near zero. Once this correction factor is updated in the memory, it will be used by the PCM under all operating conditions, open loop and closed loop. However, the values stored in the Long-term are updated only after the vehicle has entered long-term closed loop at full operating temperature. This is done to prevent any transition temperature or start-up compensation form corrupting long term fuel correction.

Long Term and Short Term Adaptive can each change the pulse width by as much as ±33% for a maximum total correction of ±66% from base pulse width calculation. Long Term Adaptive values are used during both Open Loop and Closed Loop operation.