GM Firebird 1967-1981 Repair Guide

Positive Crankcase Ventilation (PCV) System



See Figures 1, 2 and 3

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Fig. Fig. 1: Schematic of a typical closed PCV system used on V6 and V8 engines inline 6-cylinder engines similar

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Fig. Fig. 2: Hose routing and component identification for typical PCV systems used on inline 6-cylinder engines

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Fig. Fig. 3: The PCV valve on 305 and 350 (Chevrolet) engines is mounted in the rocker arm cover

In 1967, Firebirds 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 receives outside air through a vented oil filler 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. 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 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 back-fire, this arrestor prevents the spread of fire to the valve cover where it could create an explosion. Included in the system is a PCV 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 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 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, become fuel for operation, slightly increasing gas economy. In 1968, all cars were required to use the closed system and vented filler caps became a thing of the past. For more information on the PCV valve, see General Information And Maintenance .


Refer to Repair Guide for PCV system servicing.


Refer to Repair Guide for the PCV valve removal and installation procedure.

PCV Filter

Refer to Repair Guide for the PCV filter removal and installation procedure.