Long Term Fuel Trim

WARNING: This page is about a different car, the 2010 Mercury Mariner and 2010 Ford Escape. However, it is still accessible from the selected car via links, so may be relevant.

While the engine is operating in closed loop fuel, the short term fuel trim corrections can be learned by the PCM as long term fuel trim (LONGFT1) corrections. These corrections are stored in keep alive memory (KAM) in tables that are referenced by engine speed and load. 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, just like short term fuel trim, however it is not a single parameter. There is a separate long term fuel trim value that 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 or oxygenates). When viewing the LONGFT1 PID, the values may change a great deal as the engine is operated at different RPM and load points. The LONGFT1 PID displays the long term fuel trim correction that is currently being used at that RPM/load point.