GM Full Size Vans 1987-1997 Repair Guide

Crankcase Ventilation System



See Figures 1 and 2

The Positive Crankcase Ventilation (PCV) system is used to evacuate the crankcase vapors. Outside vehicle air is routed through 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.

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Fig. Fig. 1: Positive crankcase ventilation system

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Fig. Fig. 2: Cross-section of the PCV valve

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.

Other than checking and replacing the PCV valve and associated hoses, there is not 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.


See Figure 3

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Fig. Fig. 3: Check the PCV valve for vacuum at idle

With the engine running, remove the PCV from the valve cover and place your thumb over the end of the valve. Check if vacuum is present at the valve. If vacuum is not present, check for plugged hoses, blockage of the manifold port at the throttle body/carburetor unit or a faulty PCV valve. Replace as necessary. With the engine not running, remove the PCV valve from the vehicle. Shake the valve and listen for the rattle of the check valve needle. If no rattle is heard, the valve is defective and must be replaced.


See Figure 4

  1. To replace the valve, gently pull the hose from the top of the valve, then pull the valve out of the cover grommet.
  3. Installation is the reverse of removal.

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Fig. Fig. 4: Grasp the PCV valve to remove it from the grommet in the valve cover