Positive Crankcase Ventilation Valve: Description and Operation

PCV Valve Cross-Section:

DESCRIPTION
The PCV valve, consists of a needle valve, spring and housing. When the engine is "OFF", the spring holds the needle valve closed to stop vapors from entering the intake manifold. When the engine is running, manifold vacuum lifts the valve "OFF" its seat and allows crankcase vapors to enter the intake manifold. In case of an engine backfire, in the intake manifold, the valve closes. This stops the backflow of gases and prevents the ignition of fuel vapor in the crankcase.

During certain engine conditions, more blow-by gases are produced than the PCV valve can handle. In this case, the excess gases are returned through the air intake tube to the air cleaner or intake manifold, where they are burned in the combustion chamber.