GM Firebird 1967-1981 Repair Guide

Positive Crankcase Ventilation (PCV) Valve



See Figures 1, 2 and 3

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Fig. Fig. 1: Schematic of a common PCV system used on an 8-cylinder engine 6-cylinder engines similar

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Fig. Fig. 2: To remove the PCV valve, pull the valve out of the grommet in the rocker arm cover ...

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Fig. Fig. 3: ... then detach it from the vacuum hose leading to the carburetor

The Positive 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 every 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. 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 details on PCV valve operation see Driveability And Emissions Controls .


The valve is inserted into a rubber grommet in the valve cover at the large end. 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), 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 Filter See Figures 4, 5 and 6

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Fig. Fig. 4: The breather hose and filter are a part of the PCV system detach the hose from the air cleaner

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Fig. Fig. 5: The breather filter element can be removed by itself ...

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Fig. Fig. 6: ... or replace the entire filter assembly by removing the metal retaining clip (arrow)

  1. Slide the rubber coupling that joins the tube coming from the valve cover to the filter off the filter nipple. Then, remove the top of the air cleaner. Slide the spring clamp off the filter, and remove the filter.
  3. Inspect the rubber grommet in the valve cover and the rubber coupling for brittleness or cracking. Replace parts as necessary.
  5. Insert the new PCV filter through the hole in the air cleaner with open portion of the filter upward. Make sure the square portion of the filter behind the nipple fits into the (square) hole in the air cleaner.
  7. Install a new spring clamp onto the nipple. Make sure the clamp goes under the ridge of the filter nipple all the way around. Then, reconnect rubber coupling and install the air cleaner cover.