Fuel System:
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With this condition, the engine may actually run rich of stoichiometry (14.7:1 air/fuel ratio) if the Powertrain Control Module (PCM) is not able to compensate enough to correct for the condition. This situation can be caused by a fuel delivery system that is delivering excessive fuel to the engine.
Examples:
- Fuel pressure regulator causes excessive fuel pressure (system rich at all airflows)(fuel pressure can be intermittent, going to pump deadhead pressure, then returning to normal after engine is turned off then restarted).
- Fuel pressure regulator vacuum hose off (causes excessive fuel pressure at idle, system rich at idle airflows).
- Fuel pressure regulator diaphragm ruptured (fuel leaking into intake manifold, system rich at lower airflows).
- Fuel return line crimped/damaged (fuel pressure high, system rich at lower airflows).
- Fuel injector leaks (injector delivers extra fuel).
- EVAP canister purge valve leak (if canister is full of vapors, introduces extra fuel).
- Fuel rail pressure sensor (electronic returnless fuel systems) concern causes sensor to indicate lower pressure than actual. PCM commands higher pressure to the fuel pump driver module (FPDM), causing high fuel pressure (system rich at all airflows).