Positive Crankcase Ventilation: Description and Operation

Fig. 2 Positive crankcase ventilation system (typical):

NON-TURBOCHARGED GASOLINE ENGINES

This system returns blow-by gas to both the intake manifold and carburetor air cleaner, Fig. 2 (complete system schematics are located in ``Vacuum Hose Routings'').
The positive crankcase ventilation (PCV) valve is provided to conduct crankcase blow-by gas to the intake manifold.
During partial throttle operation of the engine, the intake manifold sucks the blow-by gas through the PCV valve.
Normally, the capacity of the valve is sufficient to handle any blow-by and a small amount of ventilating air.
The ventilating air is then drawn from the dust side of the carburetor air cleaner, through the tube connecting carburetor air cleaner to rocker cover, into the crankcase.
Under full-throttle condition, the manifold vacuum is insufficient to draw the blow-by flow through the valve, and its flow goes through the tube connection in the reverse direction.
On engines with an excessively high blow-by some of the flow will go through the tube connection to carburetor air cleaner under all conditions.




TURBOCHARGED GASOLINE ENGINES

This system is used to prevent blow-by gasses from entering the atmosphere, by returning the blow-by gasses created in the crankcase into the engine's intake system.
During engine operation when intake manifold is maintained, blow-by gasses flow into the intake manifold through the PCV valve located in the blow-by lower hose. When the turbocharger is in operation and positive pressure is present in the intake manifold, blow-by gasses flow through the blow-by upper hose, preventing an abnormal rise in crankcase pressure.