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 intake system (1) is supplied to the crankcase, mixed with blow by gases and then passes through a calibrated orifice in the cylinder head gasket.

The primary control is through the positive crankcase ventilation (PCV) orifice which meters the flow at a rate depending on the inlet vacuum. The PCV orifice is an integral part of the cylinder head gasket.

A plugged PCV pipe/passage way may cause: