Exhaust System Leaks
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.
In this type of condition, the engine runs rich of stoichiometric (14.7:1 air/fuel ratio) because the fuel control system is adding fuel to compensate for a perceived (not actual) lean condition. This condition is caused by the heated oxygen sensor (HO2S) sensing the oxygen (air) entering the exhaust system from an external source. The PCM reacts to this exhaust leak by increasing fuel delivery. This condition causes the exhaust gas mixture from the cylinder to be rich. Examples:
- exhaust system leaks upstream or near the HO2S
- cracked/leaking HO2S boss
- inoperative secondary air injection system