Vacuum Leaks/Unmetered Air

WARNING: This page does not describe the selected car, but rather 32 other vehicles, including the 2005 Mercury Sable, 2005 Mercury Mountaineer, 2005 Mercury Monterey, 2005 Mercury Montego, and 2005 Mercury Mariner. However, it is still accessible from the selected car via links, so may be relevant.

With this condition, the engine will run lean of stoichiometry (14.7:1 air/fuel ratio) if the PCM is not able to compensate enough to correct for the condition. This condition is caused by unmetered air entering the engine, or due to a MAF malfunction. In this situation, the volume of air entering the engine is actually greater than what the MAF sensor is indicating to the PCM. Vacuum leaks will normally be most apparent when high manifold vacuum is present (for example, during idle or light throttle). If freeze frame data indicates that the fault occurred at idle, a check for vacuum leaks/unmetered air is the best starting point.

Examples: Loose, leaking, or disconnected vacuum lines, intake manifold gaskets, or O-rings, throttle body gaskets, brake booster, air inlet tube, stuck/frozen/aftermarket PCV valve, unseated engine oil dipstick.