Chevrolet Full-size Cars 1968-1978 Repair Guide

Positive Crankcase Ventilation (PCV) Valve



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Fig. Fig. 1 Example of a common PCV system for V8 engines

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Fig. Fig. 2 Example of a common PCV system for inline 6 cylinder engines

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Fig. Fig. 3 Checking the PCV valve for vacuum

The crankcase ventilation system (PCV) must be operating properly in order to allow evaporation of fuel vapors and water from the crankcase. This system should be checked at every oil change and serviced after one year or 12,000 miles. The PCV valve is replaced after 2 years or 24,000 miles. For 1975 and later cars, the service interval has been upgraded to one year or 15,000 miles, with PCV valve replacement scheduled for two years or 30,000 miles. 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.

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 to be 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 (more gas) mixture at the carburetor. If the PCV valve or hose is clogged, this 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 it is suggested that it be replaced. If the new valve doesn't noticeably improve engine idle, the problem might be a restriction in the PCV hose. For further details on PCV valve operation see .