Positive Crankcase Ventilation (PCV)

PCV system consists of PCV valve, 3-way joint and crankcase hoses from air cleaner and rocker cover, teeing at PCV valve. PCV system draws crankcase blow-by gases (hydrocarbons) into the air induction system rather than allowing then to escape to the atmosphere. Crankcase gases are mixed with air/fuel mixture and are burned in the combustion chamber.

Crankcase ventilation system uses a PCV valve. It is basically a one-way check valve, held closed by spring pressure when engine is not running. This prevents hydrocarbon fumes from collecting in intake manifold which can result in hard starting.

When engine is running at less than idle, manifold vacuum pulls PCV valve open, allowing crankcase fumes to be drawn into intake manifold. If engine backfires through intake manifold, PCV valve closes, stopping flame travel to crankcase. This prevents ignition of fumes in crankcase.

At high RPM, when manifold vacuum is insufficient to hold PCV valve open, blow-by pressure (positive pressure) in crankcase travels from the rocker cover to the carburetor (low pressure area) and into the combustion chamber.