Positive Crankcase Ventilation: Description and Operation

Fig. 37 - Positive Crankcase Ventilation (PCV) System. Typical:

Fig. 38 - PCV Valve Cross Section:

In 1961, the earliest form of automobile emission control, the PCV system was standard equipment on all General Motors vehicles sold in California.
Two years later in 1963, all General Motors vehicles sold in the United States were equipped with this emission control system.

Combustion engines produce varying amounts of blow-by gases--gases containing undesirable pollutants which seep past the piston rings and into the crankcase during combustion.

SYSTEM OPERATION
The PCV system prevents blow-by gasses from entering the atmosphere by routing blow-by gases into the intake manifold through a vacuum controlled ventilating valve and a hose while allowing proper crankcase ventilation.
- Blow-by gases mix with the air/fuel mixture and are burned during combustion.
- When the engine is running, fresh air is drawn into the crankcase through a tube or hose connected to the air cleaner housing.

Fig. 41 - Positive Crankcase Ventilation System:

Fig. 42 - Positive Crankcase Ventilation Valve:

PCV VALVE OPERATION
The PCV valve--Fig 42--consists of needle valve, spring and housing.

When the engine is off, the spring holds the needle valve closed to stop vapors from entering the intake manifold.

When the engine is running, manifold vacuum unseats the needle valve--allowing crankcase vapors to enter the intake manifold.

In case of a backfire in the intake manifold, the valve closes--stopping the backflow and preventing ignition of fumes in the crankcase.

During certain engine conditions more blow-by gases are created than the ventilator valve can handle. The excess is returned through the air intake tube to the air cleaner and carburetor where it is burned in the engine.