Oil anywhere other than in the crankcase of an automobile is a cause for concern. In newer cars, the cause may be in the emission control system and cheap to fix. In older cars, the problem is likely more serious.
The pressure vent control (PVC) valve is part of the emission control system. It is normal for minute amounts of exhaust gas to enter the crankcase by seeping around the piston ring gaskets. If these gases have nowhere to go, pressure builds up, forcing oil out of the crankcase. For this reason, a channel exists through which excess gases can be expelled. This channel passes through a baffle into the PVC valve. When the PVC is plugged, oil escapes the system as a mist that winds up in the air filter. PVC valves are inexpensive and easy to replace.
As engines age, the pistons and rings become worn. The wear makes the parts less tight, meaning oil, fuel and exhaust gases escape the engine as mist. If this is the source of oil in an air filter, the engine needs, at a minimum, a ring job. It may also need to be rebuilt or replaced. A compression test will determine if this is the problem.
Overfilling the crankcase will cause excess blow-by, which will dirty the air filter with oil. Crankcases can become overfilled either in error (someone lost count of how many quarts they used while changing the oil) or when an unresolved engine problem is being managed by topping up the oil.
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