GM Cutlass RWD 1970-1987 Repair Guide

Crankcase Ventilation System


See Figures 1 and 2

All engines are equipped with a Positive Crankcase Ventilation (PCV) system to control crankcase blow-by vapors.

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. This valve controls the amount of gases which are recycled into the combustion chamber. At low engine speeds, the valve is partially closed by engine vacuum, 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. 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 General Information & Maintenance for PCV valve replacement intervals.

Click image to see an enlarged view

Fig. Fig. 1: PCV valve location - V6 engines

Click image to see an enlarged view

Fig. Fig. 2: PCV valve location - V8 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. Check the valve at every tune-up.

The PCV system must be operating properly in order to allow removal of fuel vapors and water from the crankcase. This system should be checked and serviced every year or 15,000 miles (24,000 km). The PCV valve should be replaced every two years or 30,000 miles (48,000 km). Normal service includes cleaning the passages of the systems hoses with solvent, inspecting them for cracks and breaks, and replacing them as necessary.

The PCV system is designed to prevent the emission of gases from the crankcase into the atmosphere. It does this by connecting a crankcase outlet (usually the valve cover) to the intake manifold with a hose. The crankcase gases travel through the hose to the intake manifold where they are returned to the combustion chambers to be burned. If maintained properly, this system reduces condensation in the crankcase and the resultant formation of harmful acids and oil dilution. A clogged PCV valve will often cause a slow or rough idle due to a richer fuel mixture. A car equipped with a PCV system has air going through a hose to the intake manifold from an outlet on the valve cover. To compensate for this extra air going to the manifold, carburetor specifications require a richer mixture at the carburetor. If the PCV valve or hose is clogged, this air doesn't go to the intake manifold which makes the mixture too rich and results in a slow, rough idle. The valve should be checked before making any carburetor adjustments. Disconnect the valve from the engine or clamp the hose shut. If the engine speed decreases less than 50 rpm, the valve is clogged and should be replaced. If the engine speed decreased much more than 50 rpm, then the valve is good. The PCV valve is an inexpensive item and it is suggested that it be replaced. If the new valve doesn't noticeably improve engine idle, the problem may be a restriction in the PCV hose.


PCV Valve
  1. To replace the valve, gently pull the hose from the top of the valve, then pull the valve out of the cover grommet.
  3. Installation is the reverse of removal.

PCV Filter
  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.