Adaptive Fuel Control Strategy

The fuel control system uses the adaptive fuel table to compensate for normal variability of the fuel system components caused by wear or aging. During closed-loop vehicle operation, if the fuel system appears "biased" lean or rich, the adaptive fuel table will shift the fuel delivery calculations to remove the bias. The fuel system monitor has two means of adapting Short Term Fuel Trim (SHRTFT) and Long Term Fuel Trim (LONGFT). SHRTFT is referred to as LAMBSE and LONGFT reference the adaptive fuel table.

Short Term Fuel Trim (SHRTFT) (displayed as SHRTFT1 and SHRTFT2 on the NGS tool) is a parameter that indicates short-term fuel adjustments. SHRTFT is commonly referred to as LAMBSE. LAMBSE is calculated by the PCM from HO2S inputs and helps maintain a 14.7:1 air/fuel ratio during closed-loop operation. This range is displayed in percentage (%). A negative percentage means that the HO2S is indicating RICH and the PCM is attempting to lean the mixture. Ideally, SHRTFT should remain near 0% but has the ability to adjust between -25% to +35%.

Long Term Fuel Trim (LONGFT) (displayed as LONGFT1 and LONGFT2 on the NGS tool) is the other parameter that indicates long-term fuel adjustments. LONGFT is also referred to as Adaptive Fuel. LONGFT is calculated by the PCM using information from the SHRTFT to maintain a 14.7:1 air/fuel ratio during closed-loop operation. The Adaptive Fuel strategy is expressed in percentages. The range of authority for LONGFT is from -35% to +35%. The ideal value is near 0% but variations of ±20% are acceptable. Information gathered at different speed load points are stored in adaptive fuel cells in the adaptive fuel tables, which can be used in the fuel calculation.

SHRTFT and LONGFT work together. If the HO2S indicates the engine is running rich the PCM will correct the rich condition by moving SHRTFT in the negative range (less fuel to correct for a rich combustion). If after a certain amount of time SHRTFT is still compensating for a rich condition, the PCM "LEARNS" this and moves LONGFT into the negative range to compensate and allows SHRTFT to return to a value near 0%.

As the fuel control and air metering components age and vary from nominal values, the adaptive fuel strategy learns corrections while in closed-loop fuel control. The corrections are stored in a table that is a function of engine speed and load. The tables reside in Keep Alive Memory (KAM) and are used to correct fuel delivery during open and closed-loop. As changing conditions continue the individual cells are allowed to update for that speed load point. If, during the adaptive process, both SHRTFT and LONGFT reach their high or low limit and can no longer compensate, the MIL is luminated and a DTC is stored.

Whenever an injector or fuel pressure regulator is replaced, KAM should be cleared. This is necessary so the fuel strategy does not use the previously learned adaptive values.