System Diagnosis - Fuel Metering System

Some failures of the fuel metering system will result in an Engine Cranks but Won't Run symptom. Referring to the Engine Cranks but Does Not Run table will determine if the problem is caused by the ignition system, PCM, or fuel pump circuit. Whenever it is determined to be a fuel problem, the Fuel System Diagnosis table will be used. This includes the fuel injectors, the fuel pressure regulator, the fuel pump, and the fuel pump relay. Whenever a problem occurs in the fuel metering system, it usually results in either a rich or lean O2S signal, which causes the PCM to change the fuel calculation (injector pulse width). The change made to the fuel calculation is indicated by a change in the short and long term fuel trim values which can be monitored by using a scan tool. A momentary change to the fuel calculation is indicated by the short term fuel trim value, while a prolonged change is indicated by the long term fuel trim value. Average fuel trim values will be around 128, but varies slightly from engine to engine.

NOTE:
- When using a scan tool to observe fuel trim values, no action is required unless a driveability symptom is present.
- Whenever both fuel trim values are fixed well above 128, see DTC P0171 or DTC P0131 tables for items which can cause a lean system. Whenever both fuel trim values are fixed well below 128, see DTC P0172 or DTC P0132 for items which can cause the system to run rich.

Listed below are examples of lean and rich conditions with the system in control and out of control.

A momentary lean O2S signal (system is in control) will appear on the scan tool as:

- Short term fuel trim value above 128 (adding fuel).
- Long term fuel trim value around 128.

A prolonged lean O2S signal (system is in control) will appear on the scan tool as:

- Short term fuel trim value around 128.
- Long term fuel trim value above 128 (added fuel).

A momentary rich O2S signal (system is in control) will appear on the scan tool as:

- Short term fuel trim value less than 128 (reducing fuel).
- Long term fuel trim value around 128.

A prolonged rich O2S signal (system is in control) will appear on the scan tool as:

- Short term fuel trim value around 128.
- Long term fuel trim value less than 128 (reduced fuel).

A prolonged rich O2S signal (system is out of control) will appear on the scan tool as:

- Short term fuel trim value much less than 128 (reducing fuel).
- Long term fuel trim value much less than 128 (reduced fuel).