Vacuum Leaks/Unmetered Air
WARNING: This page does not describe the selected car, but rather 32 other vehicles, including the 2009 Mercury Mountaineer, 2009 Mercury Milan, 2009 Mercury Mariner, 2009 Mercury Grand Marquis, and 2009 Lincoln Town Car. However, it is still accessible from the selected car via links, so may be relevant.
With this condition, the engine runs lean of stoichiometric (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 sensor concern. 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 are normally most apparent when high manifold vacuum is present (for example, during idle or light throttle). If freeze frame data indicates that the concern 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 positive crankcase valve (PCV)
- unseated engine oil dipstick