Positive Crankcase Ventilation: Description and Operation

Typical PCV System:

Positive Crankcase Ventilation (PCV) Valve:

PURPOSE
The Positive Crankcase Ventilation (PCV) system vents combustion blow-by gases from the engine crankcase into the engine air intake for burning with the normal air/fuel mixture. The PCV valve limits fresh air intake to suit engine demand and prevents combustion backfiring into the crankcase..

LOCATION
PCV valve fits in a grommet on the valve cover.

OPERATION
The ventilation valve is operated by pressure differential between the crankcase and intake manifold. With no pressure differential (the engine not operating) or when the pressure of the intake manifold is greater than that of the crankcase, the valve is closed. During engine idle, high intake manifold vacuum overcomes valve spring tension and crankcase vapors are pulled toward the intake manifold side by vacuum. This causes vapors to pass through the restricted passage between the valve and the housing. During normal engine operation, the valve remains in a position where spring pressure and intake manifold vacuum balance. Flow quantity then depends on the position of the valve. If the PCV valve becomes plugged, the system routes excess crankcase pressure and vapors back through the ventilation hose and air cleaner assembly into the induction system.