Chevrolet Full-size Cars 1968-1978 Repair Guide

Positive Crankcase Ventilation



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Fig. Fig. 1 Schematic of the closed PCV system

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Fig. Fig. 2 Components of a standard and closed type PCV system

Until 1968, Chevrolets were available with two types of crankcase ventilation systems: The closed type found on all California vehicles and some non-California vehicles, or the open type. The open ventilation system (non-California vehicles) receive outside air through a vented oil filter cap. This filtered cap permits outside air to enter the valve cover and also allows crankcase vapors to escape from the valve cover into the atmosphere. The law requires that this filler cap be non-vented to prevent the emission of vapors into the air. To supply this closed system with fresh air, a hose runs from the carburetor air cleaner to an inlet hole in the valve cover. The carburetor end of the hose fits into a cup-shaped flame arrestor and filter in the air cleaner cover. In the event of a carburetor backfire, this arrestor prevents the spread of fire to the valve cover where it could create an explosion. Included in the system is a PCV (positive crankcase ventilation) valve that fits into an outlet hole in the top of the valve cover. A hose connects this valve to a vacuum outlet at the intake manifold. Contained within the valve housing is a valve (pointed at one end, flat at the other) positioned within a coiled spring. During idle or low speed operation, when manifold vacuum is highest, the valve spring tension is overcome by the high vacuum pull and, as a result, the valve is pulled up to very nearly seal off the manifold end of the valve housing. This restricts the flow of crankcase vapors to the intake manifold at a time when crankcase pressures are lowest and least disruptive to engine performance. At times of acceleration or constant speed, intake manifold vacuum is reduced to a point where it can no longer pull against the valve spring and so, spring force pulls the valve away from the housing outlet allowing crankcase vapors to escape through the hose to the intake manifold. Once inside the manifold, the gases enter the combustion chambers to be reburned. At times of engine backfire (at the carburetor) or hen the engine is turned off, manifold vacuum ceases permitting the spring to pull the valve against the inlet (crankcase end) end of the valve housing. This seals off the inlet, thereby stopping the entrance of crankcase gases into the valve and preventing the possibility of a backfire spreading through the hose and valve to ignite these gases. The carburetor used with this system is set to provide a richer gas mixture to compensate for the additional air and gases going to the intake manifold. A valve that is clogged and stuck closed will not allow this extra air to reach the manifold. Consequently, the engine will run roughly and plugs will foul due to the creation, of an overly rich air/fuel mixture. It can be said that the PCV system performs three functions. It reduces air pollution by reburning the crankcase gases rather than releasing them to the atmosphere, and it increases engine life and gas economy. By recirculating crankcase gases, oil contamination that is harmful to engine parts is kept to a minimum. Recirculated gases returned to the intake manifold are combustible and, when combined with the air/fuel mixture from the carburetor, becomes fuel for operation, slightly increasing the economy.


Inspect the PCV system hose and connections at each tune-up and replace any deteriorated hoses. Check the PCV valve at every tune-up and replace it every 24,000 miles on 1968-74 vehicles, and every 30,000 miles on later vehicles. See for testing procedure.


The valve is inserted into a rubber grommet in the valve cover. At the narrow end, it is inserted into a hose and clamped. To remove it, gently pull it out of the valve cover, then open the clamp with a pair of pliers. Hold the clamp open while sliding it an inch or two down the hose (away from the valve), and then remove the valve. If the end of the hose is hard or cracked where it holds the valve, it may be feasible to cut the end off if there is plenty of extra hose. Otherwise, replace the hose. Replace the grommet in the valve cover if it is cracked or hard. Replace the clamp if it is broken or weak. In replacing the valve, make sure it is fully inserted in the hose, that the clamp is moved over the ridge on the valve so that the valve will not slip out of the hose, and that the valve is fully inserted into the grommet in the valve cover.