Adaptive Fuel DTC Diagnostic Techniques: Notes

The Adaptive Fuel Diagnostic Trouble Codes (DTC) Diagnostic Techniques help isolate the root cause of the adaptive fuel concern. Before proceeding, attempt to verify if any driveability concerns are present. These diagnostic aids are meant as a supplement to the pinpoint test steps. For a description of fuel trim see Fuel Trim  in this service information. Obtain Freeze Frame Data:  Freeze frame data is helpful in duplicating and diagnosing adaptive fuel concerns. The data (a snapshot of certain parameter identification (PID) values recorded at the time the DTC is stored in Continuous Memory) is helpful to determine how the vehicle was being driven when the concern occurred, and is especially useful on intermittent concerns. Freeze frame data, in many cases, helps to isolate possible areas of concern as well as rule out others. See FREEZE FRAME DATA  for a more detailed description of this data.

Using the LONGFT1 and LONGFT2 (Dual Bank Engines) PIDs:  The LONGFT1/2 PIDs are useful for diagnosing fuel trim concerns. A negative PID value indicates that fuel is being reduced to compensate for a rich condition. A positive PID value indicates that fuel is being increased to compensate for a lean condition. It is important to know that there is a separate LONGFT value that is used for each RPM/load point of engine operation. When viewing the LONGFT1/2 PIDs, the values may change a great deal as the engine is operating 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 LONGFT1/2 PIDs display the fuel trim currently being used at that RPM and load point. Observing the changes in LONGFT1/2 can help when diagnosing fuel system concerns. For example:

• A contaminated mass air flow (MAF) sensor results in matching LONGFT1/2 correction values that are negative at idle (reducing fuel), but positive (adding fuel) at higher RPM and loads.
• LONGFT1 values that differ greatly from LONGFT2 values rule out concerns that are common for both banks (for example, fuel pressure concerns, MAF sensor, etc. can be ruled out).
• Vacuum leaks result in large rich corrections (positive LONGFT1/2 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 LONGFT1/2 value) at high RPM and load.

Resetting Long Term Fuel Trims:  Long term fuel trim corrections are reset by resetting the keep alive memory (KAM). See RESETTING THE KEEP ALIVE MEMORY (KAM)  . After making a fuel system repair, reset the KAM. For example, if dirty/plugged injectors cause the engine to run lean and generate rich long term corrections, installing new injectors and not resetting the KAM causes the engine to run very rich. The rich correction eventually leans out during closed loop operation, but the vehicle may have poor driveability and high carbon monoxide (CO) emissions while it is learning.