GM Chevy Mid-Size Cars 1964-1988 Repair Guide

Positive Crankcase Ventilation (PCV) Valve


See Figures 1 and 2

Most gasoline engines covered in this guide are equipped with a Positive Crankcase Ventilation (PCV) system which must operate 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.

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Fig. Fig. 1: PCV system schematic for most gasoline V6 and V8 engines

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Fig. Fig. 2: The PCV is often grommet mounted in the valve cover on these vehicles

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 further details on PCV valve operation see Driveability & Emissions Controls of this guide.


See Figure 3

Almost all of the gasoline engines covered in this guide are equipped with a PCV valve which is mounted in a rubber grommet on one of the valve covers. Some of the earlier engines covered here may utilize a valve that is threaded into the rear of the carburetor base. Check the valve covers first, and if a valve cannot be found, look for a rubber hose that runs from the base of the carburetor to a valve cover or to the back of the manifold. If one is present, it should be a PCV valve. The valves which are threaded into the base of the carburetor are not difficult to find, but are often missed and neglected during maintenance. If your vehicle utilizes this alternate PCV valve mounting, it is even more important to check this valve's condition during maintenance as it was likely overlooked by past owners.

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Fig. Fig. 3: Separate the PCV valve from the hose, then remove the valve from the vehicle

  1. For vehicles equipped with a valve cover mounted PCV valve, grasp the valve and withdraw it from the valve cover. Holding the valve in one hand and the hose in the other, carefully pull the valve from the hose and remove from the vehicle.

Some PCV valve hoses will be retained to the valve using a clamp. If so, use a pair of pliers to slide the clamp back on the hose until it is clear of the bulged area on the end of PCV valve nipple. With the clamp in this position, the hose should be free to slip from the valve.

  1. For vehicles equipped with a carburetor mounted PCV valve, first disconnect the hose from the rear of the valve. Then carefully unthread the valve and remove it from the vehicle.

To install:
  1. Check the PCV valve for deposits and clogging. The valve should rattle when shaken. If the valve does not rattle, clean the valve with solvent until the plunger is free or replace the valve.
  3. Install the PCV hose to the grommet in the valve cover or to the back of the carburetor.
  5. Connect the PCV hose to the valve assembly.