Crankcase Ventilation Systems - Domestic: Operation

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

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

The 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.

Air mixes with blow-by gases and air/fuel mixture, and burns in combustion chamber. 

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

Air flows through valve when engine is running. 

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

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

In the event of an engine backfire through the intake manifold, the ventilator valve shuts, preventing any flow through it. This action prevents the ignition of fumes in the crankcase.

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

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