Positive Crankcase Ventilation: Description and Operation

PCV Valve SOHC:

PCV System Cross Section:

A Positive Crankcase Ventilation (PCV) system is used to consume crankcase vapors in the combustion process instead of venting 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 (PCV) into the intake manifold. The primary control is through the PCV which meters the flow at a rate depending on manifold vacuum. To maintain idle quality, the PCV restricts the flow when intake manifold 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 tube into the air cleaner to be consumed by normal combustion. The PCV valve is similar to the check valve style, but incorporates a diaphragm in place of the check valve.