PCV System: Operation
When the engine is operating, fresh air enters the positive crankcase ventilation system through the air cleaner assembly. Fresh air then flows through the crankcase breather and into the rocker arm/valve compartment.
The entering fresh air then combines with blow-by gases and unburned air/fuel mixture present in the crankcase. The combined gases are then drawn into the carburetor, through the PCV valve, by manifold vacuum. The combined gases mix with the air/fuel mixture and are burned in the combustion chamber. See Fig 1 .
The PCV valve is held closed by spring pressure when the engine is not running. This prevents hydrocarbon fumes from collecting in the intake manifold, resulting in hard starting. See Fig 2 .
When the engine is started, manifold vacuum pulls the PCV valve open against spring pressure. As long as there is engine vacuum, crankcase fumes are allowed to enter the intake manifold.
A baffle in the rocker arm cover prevents engine oil from being drawn into the intake manifold through the PCV valve.
If the engine backfires through the intake manifold, the PCV valve closes and prevents any flow of gases through it. This is to prevent the ignition of fumes in the crankcase.