Positive Crankcase Ventilation: Description and Operation

Operation Of The PCV System:

Crankcase Ventilation System:

PURPOSE
The Positive Crankcase Ventilation ([1][2]PCV) System prevents crankcase "blow-by" gases from escaping into the atmosphere.

^ "Blow-by" is the compressed gas that escapes past the cylinder-to-piston clearance. It contains high concentrations of CO and HC.

SYSTEM OPERATION
During partial throttle opening, manifold vacuum draws fresh air through a tube connected to the air cleaner into the rocker cover and crankcase, thereby drawing the blow-by gas through the PCV valve into the intake air stream.

During low vacuum conditions (full throttle or heavy engine load) the manifold vacuum is insufficient to draw the gases through the valve, so gases flow in the reverse direction, being drawn into the air cleaner and then into the intake air stream.

On engines with excessive blow-by, some of the gases will flow into the air cleaner under all conditions.

PCV VALVE OPERATION
The PCV Valve meters the flow in the system at a rate corresponding to manifold vacuum. The PCV valve also restricts the flow of crankcase emissions when vacuum is high to preserve idle stability.