GM Full Size Vans 1967-1986 Repair Guide

Positive Crankcase Ventilation



PCV is the earliest form of emission control. Prior to its use, crankcase vapors were vented into the atmosphere through a road draft tube or crankcase breather. The PCV system first appeared in 1955. Beginning 1961, the PCV system was used on all California models and in 1963 the system became standard on all models.

This system draws crankcase vapors that are formed through normal combustion into the intake manifold and subsequently into the combustion chambers to be burned. Fresh air is introduced to the crankcase by way of a hose connected to the carburetor air cleaner or a vented oil filler cap or older models. Manifold vacuum is used to draw the vapors from the crankcase through a PCV valve and into the intake manifold. Vented and nonvented filler caps were used on various models until 1968, after which only nonvented caps were used.


Other than checking and replacing the PCV valve and associated hoses, there is no other 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 1

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.

Click image to see an enlarged view

Fig. Fig. 1: Check the PCV valve for vacuum at idle


Refer to General Information & Maintenance of this repair guide for this procedure.