Using The LONGFT1 PID

The LONGFT1 PID can be useful for diagnosing fuel trim concerns. A negative PID value indicates the fuel is being reduced to compensate for a rich condition, while a positive PID value indicates the fuel is being increased to compensate for a lean condition. It is important to know there is a separate LONGFT1 value used for each RPM and load point of engine operation. When viewing the LONGFT1 PID, the value may change a great deal as the engine is operated at different RPM and load points. This is because the fuel system may have learned corrections for fuel delivery concerns that can change as a function of engine RPM and load. The LONGFT1 PID displays the fuel trim currently being used at that RPM and load point. Observing these changes in LONGFT1 can help when diagnosing fuel system concerns. For example:

• A contaminated MAF sensor results in a LONGFT1 correction value that is negative at idle (reducing fuel), but positive (adding fuel) at higher RPM and loads.
• Vacuum leaks result in large, rich corrections (positive LONGFT1 value) at idle, but little or no correction at higher RPM and loads.
• A plugged fuel filter results in no correction at idle, but large rich corrections (positive LONGFT1 value) at high RPM and load.