Positive Crankcase Ventilation Valve: Description and Operation

Fig. 15 Positive Crankcase Ventilation System:

All cars manufactured after 1966 use a positive crankcase ventilation system to prevent crankcase vapors from escaping to the atmosphere. A hose links the valve cover to either the air cleaner housing (on earlier models), or to the intake manifold. The crankcase is subjected to constant vacuum, allowing blow-by and other crankcase vapors to be drawn into the engine and burned. Baffles on the valve cover prevent lubricating oil from being drawn into the system.
Some later production vehicles employ an additional vacuum line which links the vent tube to the intake manifold, as well as the air cleaner housing. This line aids in increasing the efficiency with which vapors are extracted from the engine.