Positive Crankcase Ventilation: Description and Operation

PCV Flow (Typical):

Purpose
A Positive Crankcase Ventilation (PCV) system is used to provide more complete scavenging of crankcase vapors.

Operation
Fresh air from the air cleaner through a filter is supplied to the crankcase, mixed with blow-by gases and then passed through a PCV valve into the intake manifold.

PCV Valve (Typical):

The primary control is through the PCV valve which meters the flow at a rate depending on manifold vacuum.
To maintain idle quality, the PCV valve 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.