Fuel Trim

PURPOSE
The fuel control system uses the fuel trim 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 fuel trim table will shift the fuel delivery calculations to remove the bias. The fuel system monitor has two means of adapting - Short Term Fuel Trim (FT) and Long Term Fuel Trim. Short Term FT is referred to as LAMBSE and Long Term FT reference the fuel trim table.

SHORT TERM FUEL TRIM
Short Term Fuel Trim (displayed as SHRTFT1 and SHRTFT2 on the New Generation STAR [NGS] tool) is a parameter that indicates short-term fuel adjustments. Short Term FT 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 Powertrain Control Module (PCM) is attempting to lean the mixture. Ideally, Short Term FT should remain near 0% but has the ability to adjust between -25% to +35%.

LONG TERM FUEL TRIM
Long Term Fuel Trim (displayed as LONGFT1 and LONGFT2 on the NGS tool) is the other parameter that indicates long-term fuel adjustments. Long Term FT is also referred to as Fuel Trim. Long Term FT is calculated by the PCM using information from the Short Term FT to maintain a 14.7:1 air/fuel ratio during closed loop operation. The Fuel Trim strategy is expressed in percentages. The range of authority for Long Term FT 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 fuel trim cells in the fuel trim tables, which can be used in the fuel calculation.

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

As the fuel control and air metering components age and vary from nominal values, the fuel trim 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 Random Access Memory (RAM) 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 Short Term FT and Long Term FT reach their high or low limit and can no longer compensate, the Malfunction Indicator Lamp (MIL) is illuminated and a Diagnostic Trouble Code (DTC) is stored.

Whenever a fuel injector or fuel pressure regulator is replaced, Keep Alive RAM should be cleared. This is necessary so the fuel strategy does not use the previously learned fuel trim values.

To clear Keep Alive RAM, refer to Testing and Inspection / Procedures / Powertrain Control Module (PCM) Reset (Clearing DTCs).