GM Cavalier/Sunbird/Skyhawk/Firenza 1982-1994

Crankcase Ventilation System



See Figures 1, 2 and 3

A crankcase ventilation system is used on all vehicles to evacuate the crankcase vapors. There are 2 types of ventilation systems: Crankcase Ventilation (CV) and Positive Crankcase Ventilation (PCV). Both systems purge crankcase vapors and differ only in the use of fresh air.

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Fig. Fig. 1: The PCV system circulates crankcase vapors into the intake manifold for burning - 1990 2.0L (VIN M) engine shown

The CV system, used on the 3.1L engine, introduces fresh air into the crankcase.

The PCV system and the CV system on the 3.1L engine, circulates fresh air from the air cleaner or intake duct through the crankcase, where it mixes with blow-by gases and then passes through the Positive Crankcase Ventilation (PCV) valve or constant bleed orifice into the intake manifold.

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

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Fig. Fig. 3: Crankcase Ventilation (CV) flow - 3.1L engine shown

When manifold vacuum is high, such as at idle, the orifice or valve restricts the flow of blow-by gases into the intake manifold. If abnormal operating conditions occur, the system will allow excessive blow-by gases to back flow through the hose into the air cleaner. These blow-by gases will then be mixed with the intake air in the air cleaner instead of the manifold. The air cleaner has a small filter attached to the inside wall that connects to the breather hose to trap impurities flowing in either direction.

A plugged PCV valve, orifice or hose may cause rough idle, stalling or slow idle speed, oil leaks, oil in the air cleaner or sludge in the engine. A leak could cause rough idle, stalling or high idle speed. The condition of the grommets in the valve cover will also affect system and engine performance.


PCV System
  1. Run the engine at idle at normal operating temperature.
  3. Remove the PCV valve or orifice from the grommet in the valve cover and place your thumb over the end to check if vacuum is present. If vacuum is not present, check for plugged hoses or manifold port. Repair or replace as necessary.
  5. If the engine is equipped with a PCV valve, stop the engine and remove the valve. Shake and listen for the rattle of the check valve needle. If no rattle is heard, replace the valve.

CV System
  1. Check the CV system for proper flow by looking for oil sludging or leaks.
  3. If noted, check the smaller nipple of the oil/air separator by blowing through it or inserting a 0.06 in. (1.52mm) plug gauge into the orifice inside the nipple.
  5. If the orifice is plugged, replace the CV oil/air separator assembly.


Removal and installation of the PCV or CV valve is located in Routine Maintenance of this repair guide.