A positive crankcase ventilation system is used to consume crankcase vapors in the combustion process instead of venting them to atmosphere. Fresh air from the throttle body is supplied to the crankcase, mixed with blow-by gases and then passed through a positive crankcase ventilation valve (3) into the intake manifold.

The primary control is through the positive crankcase ventilation valve which meters the flow at a rate depending on intake vacuum. To maintain idle quality, the positive crankcase ventilation valve restricts the flow when intake 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: