GM Full-Size Trucks 1980-1987 Repair Guide

Positive Crankcase Ventilation



See Figures 1, 2, 3, 4 and 5

The Positive Crankcase Ventilation (PCV) system is used to evacuate the crankcase of vapors. Outside air is routed through a breather in the air cleaner to the crankcase where it mixes with the blow-by gases and is passed through the PCV valve. It is then routed into the intake manifold. The PCV valve meters the air flow rate which varies under engine operation depending on manifold vacuum. In order to maintain idle quality, the PCV valve limits the air flow when intake manifold vacuum is high. If abnormal operating conditions occur, the system will allow excessive blow-by gases to back flow through the crankcase vent tube into the air cleaner. These blow-by gases will then be burned by normal combustion.

A plugged PCV valve or hose may cause rough idle, stalling or slow idle speed, oil leaks, oil in the air cleaner or sludge in the engine. A leaking PCV valve or hose could cause rough idle, stalling or high idle speed.

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Fig. Fig. 1: PCV System

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Fig. Fig. 2: Closed and positive PCV systems, 6-cylinder shown

Other than checking and replacing the PCV valve, breather and associated hoses, there is no service required. Engine operating conditions that would direct suspicion to the PCV system are rough idle, oil present in the air cleaner, oil leaks and excessive oil sludging or dilution. If any of the above conditions exist, remove the PCV valve and shake it. A clicking sound indicates that the valve is free. If no clicking sound is heard, replace the valve. Inspect the PCV breather in the air cleaner. Replace the breather if it is so dirty that it will not allow gases to pass through. Check all the PCV hoses for condition and tight connections. Replace any hoses that have deteriorated.

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Fig. Fig. 3: To remove the breather from the air cleaner you must first disconnect the valve cover hose

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Fig. Fig. 4: Once the hose is disconnected, remove the C-clip and the breather can be pulled from the air cleaner assembly


  1. Remove PCV valve from the rocker arm cover. Do not disconnect the vacuum hose from the valve.

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Fig. Fig. 5: V8 PCV valve location

  1. Start the engine and allow it to reach normal operating temperature. Continue to run the engine at idle.
  3. Place your thumb over the end of the valve to check if vacuum is present at the valve. If vacuum is not present, check for plugged hoses or blockage at the manifold port or a faulty PCV valve. Replace as necessary.