Positive Crankcase Ventilation: Description and Operation

PCV Valve (Typical):

PCV Flow (Typical):

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

The primary control is through the valve which meters the flow at a rate depending on manifold vacuum.

To maintain idle quality, the crankcase ventilation 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.

A leaking valve or hose would cause:
^ Rough idle.
^ Stalling.
^ High idle speed.

RESULTS OF INCORRECT OPERATION
A plugged valve or hose may cause:
^ Rough idle.
^ Stalling or slow idle speed.
^ Oil leaks.
^ Oil in air cleaner.
^ Sludge in engine.

An engine which is operated without any crankcase ventilation can be damaged. Therefore, it is important to replace the crankcase ventilation valve and air cleaner breather at regular intervals.