A crankcase ventilation system is used to consume crankcase vapors in the combustion process instead of venting them to the atmosphere. Fresh air from the throttle body is supplied to the crankcase, mixed with blow-by gases, then passed through the PCV valve into the intake manifold.

The primary control is through the positive crankcase ventilation (PCV) valve which meters the flow at a rate depending on the inlet vacuum. To maintain idle quality, the PCV valve restricts the flow when the inlet vacuum is high. If abnormal operating conditions arise, the system is designed to allow excessive amounts of blow-by gases to back flow through the crankcase vent into the throttle body to be consumed by normal combustion.

A plugged valve may cause the following conditions:

A leaking valve would cause