GM Full-Size Trucks 1970-1979 Repair Guide

Positive Crankcase Ventilation


See Figure 1

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Fig. Fig. 1: PCV system schematic

This system draws crankcase vapors that are formed through normal combustion into the intake manifold and 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. Manifold vacuum is used to draw the vapors from the crankcase through a PCV valve and into the intake manifold.


See Figure 2

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

The PCV system should be inspected as stated in the Maintenance Interval Chart in Routine Maintenance . Other than checking and replacing the PCV valve and associated hoses, there is no other service required.

  1. Remove the PCV valve from the intake manifold or valve cover.
  3. Allow the engine to idle.
  5. Place your thumb over the end of the valve to check for vacuum. If there is no vacuum at the valve, check for plugged hoses or valve.
  7. Remove the valve and shake it. If it rattles the valve is still good, if not, replace it.
  9. After installation of a PCV valve or hoses adjust the idle if necessary.