The Positive Crankcase Ventilation (PCV) system returns combustible gases which have leaked past the piston rings into the crankcase, back through the intake manifold for reburning. This leakage is the normal result of the necessary working clearance of the piston rings. If these gases were to remain in the crankcase they would react with the oil to form sludge. Since there is also a certain amount of unburned fuel in the gases, dilution of the oil would occur. Both sludge and diluted oil will accelerate the wear of the engine.
Along with gases returning to the intake manifold, the PCV valve also returns a certain amount of additional air. The carburetor used with this system has been calibrated to compensate for the additional air intake.
The system consists of a hose connecting the air cleaner to the cam cover and another hose connecting the PCV valve mounted in a grommet in the cam cover and the intake manifold.
The PCV valve regulates the flow of combustion gases through the system. During engine idle and deceleration when intake manifold vacuum is high, the PCV valve restricts vapor flow to the intake manifold. When the engine is accelerated or is at constant speed, intake manifold vacuum is low and the PCV valve allows crankcase gases to flow into the intake manifold. Should the engine backfire, the plunger inside the valve is forced against its seat preventing the backfire from traveling through the PCV valve and into the engine crankcase.
The PCV valve is checked for proper operation simply by removing it from the grommet in the cam cover and shaking. If the plunger in the valve rattles, the valve is good and can be replaced. The PCV valve can also be checked for vacuum at idle. At the required mileage intervals, install a new PCV valve and use compressed air to blow out the PCV valve hose to eliminate any restrictions.