Long Term Fuel Trim

WARNING: This page does not describe the selected car, but rather 32 other vehicles, including the 2005 Mercury Sable, 2005 Mercury Mountaineer, 2005 Mercury Monterey, 2005 Mercury Montego, and 2005 Mercury Mariner. However, it is still accessible from the selected car via links, so may be relevant.

While the engine is operating in closed loop fuel control, the short term fuel trim corrections are learned by the PCM as LONGFT1 and 2 corrections. These corrections are stored in the KAM fuel trim tables. Fuel trim tables are based on engine speed and load and by bank for engines with 2 HO2S sensors forward of the catalyst. Learning the corrections in KAM improves both open loop and closed loop air/fuel ratio control. Advantages include:

Short term fuel trim does not have to generate new corrections each time the engine goes into closed loop.
Long term fuel trim corrections can be used both while in open loop and closed loop modes.

Long term fuel trim is represented as a percentage, similar to the short term fuel trim, however it is not a single parameter. A separate long term fuel trim value is used for each RPM/load point of engine operation. Long term fuel trim corrections may change depending on the operating conditions of the engine (RPM and load), ambient air temperature, and fuel quality (% alcohol, oxygenates). When viewing the LONGFT1/2 PID(s), the values may change a great deal as the engine is operated at different RPM and load points. The LONGFT1/2 PID(s) will display the long term fuel trim correction that is currently being used at that RPM/load point.