PCV System: Operation

Air is supplied to the crankcase ventilation system through a crankcase ventilating filter assembly, located in air cleaner housing or on rocker arm cover.

When engine is operating, fresh air enters crankcase ventilation system through the air cleaner and filter.

Air then flows into the rocker arm cover and valve compartment. It combines with blow-by gas and unburned air/fuel mixture and burns in combustion chamber. See Fig 1 .

Fig 1: Crankcase Ventilation System (Typical) Air mixes with blow-by gases and air/fuel mixture, then burns in combustion chamber.

Ventilator valve is held closed by spring pressure when engine is not running. See Fig 2 . This prevents accumulation of hydrocarbon fumes from collecting in intake manifold, which could result in hard starting.

Fig 2: PCV Valve & Airflow (Typical) Air flows through valve when engine is running.

When engine is started, manifold vacuum pulls valve open against spring pressure. As long as there is engine vacuum, the valve floats, permitting crankcase fumes to enter intake manifold.

A baffle in rocker arm cover prevents oil from being drawn into intake manifold through ventilator valve.

If the engine backfires, the ventilator valve will close. This will prevent ignition of fumes in crankcase.

During certain engine operations, more blow-by is created than ventilator valve can handle. Excess blow-by is returned to air cleaner and carburetor through rocker arm cover and breather assembly. It is then burned in the combustion chamber.

A breather assembly acts as separator to keep oil from being drawn into air cleaner during this operation.