Positive Crankcase Ventilation: Description and Operation

Fig. 9 Positive crankcase ventilation system.:

This system, Fig. 9, is a positive crankcase system and is designed to prevent blow-by gas from escaping to the atmosphere. The positive crankcase ventilation (PCV) valve contains a spring loaded plunger. When the engine starts, the plunger in the PCV valve is raised in proportion to intake manifold vacuum and the blow-by gas is drawn directly into the intake manifold.
When the throttle valves are closed or partly open, blow-by vapor is returned directly to the intake manifold through the breather chamber and PCV valve.
When the throttle valves are wide open, intake manifold vacuum decreases and vacuum in the air cleaner assembly increases. The PCV valve then increases the blow-by vapor flow. A small amount of vapor is returned through the valve cover breather hose and into the air cleaner.