Fuel System

WARNING: This page is about a different car, the 2010 Mercury Mariner and 2010 Ford Escape. However, it is still accessible from the selected car via links, so may be relevant.

With this condition, the engine may actually run rich of stoichiometry (14.7:1 air/fuel ratio) if the 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.

For example,

EVAP canister purge valve leak (if canister is full of vapors, introduces extra fuel).
fuel injector leaks (injector delivers extra fuel).
fuel pressure regulator causes excessive fuel pressure (system rich at all airflows), fuel pressure is intermittent, going to pump deadhead pressure, then returning to normal after the engine is turned off and restarted)