GM Caprice 1990-1993 Repair Guide

Crankcase Ventilation System


See Figures 1, 2, 3 and 4

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Fig. Fig. 1: PCV system operation - V6 & V8 engines

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Fig. Fig. 2: Positive Crankcase Ventilation (PCV) system - 5.0L (VIN Y) engine

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Fig. Fig. 3: Positive Crankcase Ventilation (PCV) system - except 5.0L (VIN Y) engine

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Fig. Fig. 4: Cross-section of standard PCV valve


The positive crankcase ventilation (PCV) system is used to control crankcase blow-by vapors. The system functions as follows:

The crankcase (blow-by) gases are recycled in the following way:

As the engine is running, clean, filtered air is drawn through the air filter and into the crankcase. As the air passes through the crankcase, it picks up the combustion gases and carries them out of the crankcase, through the PCV valve, and into the induction system. As they enter the intake manifold, they are drawn into the combustion chamber where they are reburned.

The most critical component in the system is the PCV valve. This valve controls the amount of gases which are recycled into the combustion chamber. At low engine speeds, the valve is partially closed, limiting the flow of gases into the intake manifold. As engine speed increases, the valve opens to admit greater quantities of gases into the intake manifold. If the PCV valve becomes clogged, the system is designed to allow excessive amounts of blow-by gases to flow back through the crankcase tube and into the air cleaner where they become consumed by normal combustion.


Inspect the PCV system hose and connections at each tune-up and replace any deteriorated hoses. Check the PCV valve at every tune-up and replace it at 30,000 mile intervals.


A good indication of a faulty or clogged PCV system is generally represented by external oil leaks or oil being pushed back up through the crankcase ventilation tube into the air cleaner. Remember that the purpose of the PCV system is two-fold, one is to relieve the crankcase of excessive vapors and the other is to relieve excessive pressure caused by normal amounts of piston ring blow-by.

  1. Remove the PCV valve from the intake manifold or valve cover.
  3. Run the engine at idle.
  5. Place your thumb over the end of the valve. Check for vacuum. If there is no vacuum at the valve, check for plugged valve or vacuum lines.
  7. Shut off the engine. Shake the valve and listen for the rattle. If valve doesn't rattle, replace it.
  9. Reinstall the valve and start the engine.
  11. Remove the oil dipstick. Connect a vacuum gauge to the dipstick tube.
  13. Remove the PCV fresh vent hose at the air cleaner and plug the opening in the hose.
  15. Run the engine at 1500 rpm for approximately 30 seconds. Verify there is vacuum present.
  17. If there is no vacuum, inspect the valve cover, oil pan and intake manifold gaskets for leaks. If the gauge is showing pressure or pushes the gauge out of the dipstick tube, inspect the PCV valve or hoses for restriction; also inspect the engine for excessive blow-by.