Fuel Trim

Fuel control system uses a fuel trim table to compensate for normal variability of fuel system components caused by wear or aging. During closed loop vehicle operation, if fuel system appears "biased" (lean or rich), the fuel trim table will shift fuel delivery calculations to remove bias. Fuel system monitor has 2 means of adapting Short Term Fuel Trim (FT) and Long Term Fuel Trim (FT). Short Term FT is referred to as LAMBSE and Long Term FT references the fuel trim table in Keep Alive Random Access Memory (RAM).

Short Term FT is displayed as SHRTFT1 and SHRTFT2 on scan tool and is a parameter that indicates short-term fuel adjustments. Short Term FT is commonly referred to as LAMBSE. LAMBSE is calculated by PCM from Heated Oxygen Sensor (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 HO2S is indicating RICH and PCM is attempting to lean the mixture. Ideally, Short Term FT may remain near zero percent but can adjust between -25 to +35 percent.

Long Term FT is displayed as LONGFT1 and LONGFT2 on scan tool and 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 PCM using information from Short Term FT to maintain a 14.7:1 air/fuel ratio during closed loop operation. Fuel Trim strategy is expressed in percentages. The range of authority for Long Term FT is from -35 to +35 percent. Ideal value is near zero percent, but variations of plus or minus 20 percent are acceptable. Information gathered at different speed load points are stored in fuel trim cells in fuel trim tables, which can be used in fuel calculation.

Short Term FT and Long Term FT work together. If HO2S indicates engine is running rich, PCM will correct 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, PCM "learns" this and moves Long Term FT into negative range to compensate and allows Short Term FT to return to a value near zero percent.

As fuel control and air metering components age and vary from nominal values, fuel trim learns corrections while in closed loop fuel control. Corrections are stored in a table that is a function of engine speed and load. 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, individual cells are allowed to update for that speed load point. If both Short Term FT and Long Term FT reach their high or low limit and can no longer compensate during adaptive process, MIL is illuminated and a DTC is stored. Whenever a fuel injector or fuel pressure regulator is replaced, RAM should be cleared. This is necessary so PCM does not use previously learned fuel trim values. To clear RAM, see KEEP ALIVE RANDOM ACCESS MEMORY RESET PROCEDURE under CLEARING CODES under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - EEC-V - GASOLINE & NGV article.