Fuel Metering System

FUEL METERING SYSTEM
Some failures of this system will result in an engine cranks but won't run symptom. If this condition exists, see TABLE A-3. This Table will determine if the problem is caused by the ignition system, PCM, or fuel pump circuit. If it is determined to be a fuel problem, TABLE A-6 will be used. This includes the fuel injectors, fuel pressure regulator, fuel pump, and fuel pump relay. The fuel system wiring schematic diagram is covered on the facing page of Table A-3 and A-5.
If 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 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 vary slightly from engine to engine.

Important: When using a scan tool to observe fuel trim values, remember that if the system is in control, no action is required unless a driveability symptom is present.

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 prolonged lean O2S signal (system is out of control) will appear on the scan tool as
- Short term fuel trim value well above 128 (adding fuel).
- Long term fuel trim value well above 128 (added fuel).

If both fuel trim values are fixed well above 128, see DTC P0172 for items which can cause a lean system.

- 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 (reduced1 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).

Whenever both fuel trim values are fixed well below 128, see DTC P0171 for items which can cause the system to run rich.