Chevrolet Full Size Cars 1979-1989

Positive Crankcase Ventilation



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Fig. Fig. 1 Cross sectional view of a PCV valve

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Fig. Fig. 2 Exploded view of a common crankcase ventilation system

Before the years covered by this guide, Federal law mandated that oil fill caps on engines be sealed to prevent the escape of crankcase vapors into the atmosphere. The result was a closed crankcase ventilation system which is designed to control crankcase pressure, while preventing it from escaping into the atmosphere.

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.

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. The restricted flow also prevents the crankcase vapors from altering the idle mixture and thereby disrupting idle quality. 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 different 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 increases 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.


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 valve and hose may be checked by removing the valve from the grommet and feeling for vacuum while the engine is running. If vacuum is present, the valve and hose are likely in good condition.

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. On carbureted vehicles, to compensate for this extra air going to the manifold, carburetor specifications require a richer mixture (more gas). 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. 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 the PCV system, please refer to of this guide.


PCV Valve

The large end of 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.

PCV Breather

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Fig. Fig. 3 If the PCV breather is being completely removed from the air cleaner, disconnect the hose

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Fig. Fig. 4 On some applications, the element may be removed from the plastic housing for cleaning or replacement

Most breathers are located inside the air cleaner assembly, though they may be mounted directly to a valve cover. Although a breather may in some cases be removed and cleaned, it is an inexpensive part and it is wise to replace it if dirty. Breathers which are mounted directly to the valve cover may be simply grasped and pulled from the cover grommet. For breathers which are mounted inside the air cleaner follow the procedure listed below.

  1. Loosen the wing nut and remove the top of the air cleaner assembly.
  3. Slide the rubber coupling that joins the tube coming from the valve cover to the breather off the breather nipple.
  5. Slide the spring clamp off the breather nipple which is protruding from the air cleaner housing, then withdraw the breather from inside the air cleaner assembly.
  7. Inspect the rubber grommet in the valve cover and the rubber coupling for brittleness or cracking. Replace parts as necessary.

To install:

  1. Insert the new PCV breather through the hole in the air cleaner with the open portion of the breather upward. Make sure that the square portion of breather behind the nipple fits into the (square) hole in the air cleaner.
  3. Install a new spring clamp onto the nipple. Make sure the clamp goes under the ridge on the breather nipple all the way around.
  5. Reconnect the rubber coupling.
  7. Install the air cleaner cover and tighten the wing nut.