POSITIVE CRANKCASE VENTILATION (PCV) SYSTEM

The positive crankcase ventilation system uses manifold vacuum to purge the crankcase blow-by fumes. The fumes are then aspirated back into the engine where they are reburned.

A hose is tapped into the crankcase at a point that is well above the engine oil level. The other end of the hose is tapped into the intake manifold or the base of the carburetor.

NOTE

If the hose is tapped into the carburetor base, it will be in a location that is between the throttle valves and the intake manifold so that it will receive manifold vacuum.

An inlet breather is installed on the crankcase in a location that is well above the level of the engine oil. The inlet breather also is located strategically to ensure complete purging of the crankcase fresh air. The areas of the crankcase where the vacuum hose and inlet breather are tapped have baffles to keep motor oil from leaving the crankcase.

A flow control valve is installed in the line that connects the crankcase to the manifold. It is called a positive crankcase ventilation (PCV) valve (fig. 4-52) and serves to avoid the air-fuel mixture by doing the following:

• Any periods of large throttle opening will be accompanied by heavy engine loads. Crankcase blow-by will be at its maximum during heavy engine loads. The PCV valve will react to the small amount of manifold vacuum that also is present during heavy engine loading by opening fully through the force of its control valve spring. In this way, the system provides maximum effectiveness during maximum blow-by periods.
• Any period of small throttle opening will be accompanied by small engine loads, high manifold vacuum, and a minimum amount of crankcase blow-by. During these periods, the high manifold vacuum will pull the PCV valve to its position of minimum opening. This is important to prevent an excessively lean air-fuel mixture.
• In the event of engine backfire (flame traveling back through the intake manifold), the reverse pressure will push the rear shoulder of the control valve against the valve body. This will seal the crankcase from the backfire which could otherwise cause an explosion. The positive crankcase ventilation system can be either the open or closed type (fig. 4-52).
• The open type has an inlet breather that is open to the atmosphere. When this system is used, it is possible for a portion of the crankcase blow-by to escape through the breather whenever the engine is under a sustained heavy load.
• The closed type has a sealed breather that is connected to the air filter by a hose. Any blow-by gases that escape from the breather when this system is used will be aspirated into the carburetor and reburned.