GM Chevy Mid-Size Cars 1964-1988 Repair Guide

Positive Crankcase Ventilation



See Figures 1, 2 and 3

Until 1967, most mid-size Chevrolets were available with two types of crankcase ventilation systems: The closed type found on all California cars and some non-California cars, or the open type. The open ventilation system (non-California cars) receives outside air through a vented oil filler cap was usually equipped with a PCV valve that is threaded into the base of the carburetor. The vented oil cap permits outside air to enter the crankcase, but also allows crankcase vapors to escape from the valve cover into the atmosphere.

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Fig. Fig. 1: Open (top) and closed PCV systems

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Fig. Fig. 2: PCV system schematic for a common V8 engine (most closed systems similar)

The 1967 California law and later Federal 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 breather 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 breather 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.

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Fig. Fig. 3: Cross sectional view of a PCV valve

Included in the closed 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 PCV housing is a unidirectional 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 when 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 is 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, it increases engine life and it increase fuel 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 fuel economy. In 1968, all cars were required to use the closed system and vented filler caps became a thing of the past.


See Figure 4

Normal service entails cleaning the passages of the system hoses with solvent, inspecting them for cracks and breaks, and replacing them as necessary. The PCV valve contains a check valve and, when working properly, this valve will make a rattling sound when the outside case is tapped. If it fails to rattle, then it is probably stuck in a closed position and needs to be replaced.

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Fig. Fig. 4: The valve and hose may be checked by removing the valve from the grommet and feeling for vacuum while the engine is running

The PCV system is designed to prevent the emission of gases from the crankcase into the atmosphere. It does this by connecting a crankcase outlet (valve cover, oil filler tube, back of engine) to the intake manifold with a hose. The crankcase gases travel through the hose to the intake manifold where they are returned to the combustion chamber and burned. If maintained properly, this system reduces condensation in the crankcase and the resultant formation of harmful acids and oil dilution. A clogged PCV valve will often cause a slow or rough idle due to a richer fuel mixture. A car equipped with a PCV system has air going through a hose to the intake manifold from an outlet at the valve cover, oil filler tube, or rear of the engine. To compensate for this extra air going to the manifold, carburetor specifications require a richer mixture (more gas) at the carburetor. If the PCV valve or hose is clogged, the additional air doesn't go to the intake manifold and the fuel mixture is too rich. A rough, slow idle results. The valve should be checked before making any carburetor adjustments. Disconnect the valve from the engine or merely clamp the hose shut. If the engine speed decreases less than 50 rpm, the valve is clogged and should be replaced. If the engine speed decreases much more than 50 rpm, then the valve is good. The PCV valve is an inexpensive item and is easily replaced during routine maintenance. If the new valve doesn't noticeably improve engine idle, the problem might be a restriction in the PCV hose. For more information regarding testing and service intervals, please refer to Routine Maintenance of this guide.


See Figure 5

The valve is inserted into a rubber grommet in the valve cover at the large end (on 1964-66 V8 engines, the valve is located in, or near the carburetor base). 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.

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