GM Buick/Oldsmobile/Pontiac/Full-Size 1975-1990 Repair Guide

Crankcase Ventilation Systems





See Figures 1 and 2

All Buick, Oldsmobile, Chevrolet and Pontiac gasoline engines covered in this guide are equipped with a positive crankcase ventilation (PCV) system to control crankcase blow-by vapors. The system functions as follows:

When the engine is running, a small portion of the gases which are formed in the combustion chamber leak by the piston rings and enter the crankcase. Since these gases are under pressure, they tend to escape from the crankcase and enter the atmosphere. If these gases are allowed to remain in the crankcase for any period of time, they contaminate the engine oil and cause sludge to build up in the crankcase. If the gases are allowed to escape into the atmosphere, they pollute the air with unburned hydrocarbons. The job of the crankcase emission control equipment is to recycle these gases back into the engine combustion chamber where they are reburned.

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 oil separator, 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. Located in the valve cover or intake manifold, 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 the gases into the intake manifold. As engine speed increases, the valve opens to admit greater quantities of the gases into the intake manifold. As engine speed increases, the valve opens to admit greater quantities of the gases into the intake manifold. If the valve should become blocked or plugged, the gases will be prevented from escaping from the crankcase by the normal route. Since these gases are under pressure, they will find their own way out of the crankcase. This alternate route is usually a weak oil seal or gasket in the engine. As the gas escapes by the gasket, it also creates an oil leak. Besides causing oil leaks, a clogged PCV valve also allows these gases to remain in the crankcase for an extended period of time, promoting the formation of sludge in the engine. See Routine Maintenance for PCV valve replacement intervals.

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Fig. Fig. 1: V6 PCV valve location

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Fig. Fig. 2: V8 PCV valve location

Diesel Engines

See Figure 3

A Crankcase Depression Regulator Valve (CDRV) or flow control valve is used to regulate (meter) the flow of crankcase gases back into the engine to be burned. The CDRV is designed to limit vacuum in the crankcase as the gases are drawn from the valve covers through the CDRV and into the intake manifold (air crossover).

Fresh air enters the engine through the combination filter, check valve and oil fill cap. The fresh air mixes with blow-by gases and enters both valve covers. The gases pass through a filter installed on the valve covers and are drawn into connecting tubing.

Intake manifold vacuum acts against a spring loaded diaphragm to control the flow of crankcase gases. Higher intake vacuum levels pull the diaphragm closer to the top of the outlet tube. This reduces the amount of gases being drawn from the crankcase and decreases the vacuum level in the crankcase. As the intake vacuum decreases, the spring pushes the diaphragm away from the top of the outlet tube allowing more gases to flow to the intake manifold.

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Fig. Fig. 3: Diesel crankcase ventilation flow


Gasoline Engines

Slow, unstable idling, frequent stalling, oil leaks and oil in the air cleaner are all signs that the PCV valve may be clogged or faulty. Follow the PCV valve testing procedure in Routine Maintenance and replace the valve if necessary. Check the valve at every tune-up.

Diesel Engines

Do not attempt to test these valves. Instead, follow the cleaning procedures in Routine Maintenance if you are experiencing problems with the system.


Gasoline Engines
  1. To replace the filter, slide the rubber coupling that joins the tube coming from the valve cover to the filter off the filter nipple. Then, remove the top of the air cleaner. Slide the spring clamp off the filter, and remove the filter.
  3. Inspect the rubber grommet in the valve cover and the rubber coupling for brittleness and cracking. Replace parts as necessary.
  5. Insert the new PCV filter through the hole in the air cleaner with the open portion of the filter upward. Make sure that the square portion of filter behind the nipple fits into the (square) hole in the air cleaner.
  7. Install a new spring clamp onto the nipple. Make sure the clamp goes under the ridge on the filter nipple all the way around. Then, reconnect the rubber coupling and install the air cleaner cover.
  9. To replace the valve, gently pull the hose from the top of the valve, then pull the valve out of the cover grommet.
  11. Installation is the reverse of removal.

Diesel Engines

The filter assemblies can be removed by carefully prying them from the valve covers.