GM Bonneville/Eighty Eight/LeSabre 1986-1999

Crankcase Ventilation System



See Figures 1 through 5

All vehicles are equipped with a Positive Crankcase Ventilation (PCV) or Crankcase Ventilation (CV) 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 or supercharger inlet, they are drawn into the combustion chamber where they are reburned.

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Fig. Fig. 1: Typical PCV system air flow schematic - early model vehicles

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 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.

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Fig. Fig. 2: Cross-sectional view of a PCV valve

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.

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Fig. Fig. 3: Crankcase ventilation flow - 3.8L (VIN L) engine

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Fig. Fig. 4: View of the crankcase ventilation system flow - 3.8L (VIN 1) engine

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Fig. Fig. 5: Schematic of the crankcase ventilation air flow - 3.8L (VIN K) engine

Other than checking and replacing the PCV valve and associated hoses, there is no service required. Engine operating conditions that would direct suspicion to the PCV system are rough idle, oil present in the air cleaner, oil leaks and excessive oil sludging or dilution. If any of the above conditions exist, remove the PCV valve and shake it. A clicking sound indicates that the valve is free. If no clicking sound is heard, replace the valve. Inspect the PCV breather in the air cleaner. Replace the breather if it is so dirty that it will not allow gases to pass through. Check all the PCV hoses for condition and tight connections. Replace any hoses that have deteriorated.


PCV Valve

See Figures 6, 7 and 8

If the engine is idling rough, check for a obstructed PCV valve or a restricted hose.

  1. Start the engine.
  3. With the engine at normal operating temperature, run at idle.
  5. Remove the PCV valve or orifice from the grommet in the valve cover or manifold and place 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.

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Fig. Fig. 6: Check the PCV valve for vacuum at idle

  1. Stop the engine and remove the valve. Shake the valve and listen for the rattle of the check valve needle. If there is no rattle heard when the valve is shaken, replace the valve.

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Fig. Fig. 7: Remove the valve from the manifold and check for vacuum - 3.8L (VIN C) engine shown

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Fig. Fig. 8: Remove and shake the PCV valve; if a rattling noise is heard, the valve is OK


See Figure 9

If the engine is idling rough, check for a plugged PCV valve, as follows:

  1. Remove the PCV valve from the intake manifold.
  3. Shake the valve. If you can hear rattling from the needle inside the valve, the valve is OK. If no rattle is heard, replace the PCV valve.

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Fig. Fig. 9: Exploded view of the PCV valve mounting - 1992 3.8L engine shown

PCV System
  1. Check to make sure the engine has the correct PCV valve or bleed orifice.
  3. Start the engine and bring to normal operating temperature.
  5. Block off the PCV system fresh air intake passage.
  7. Remove the engine oil dipstick and install a vacuum gauge on the dipstick tube.
  9. Run the engine at 1500 rpm for 30 seconds then read the vacuum gauge with the engine at 1500 rpm.
  11. If vacuum is present, the PCV system is functioning properly.
  13. If there is no vacuum, the engine may not be sealed and/or is drawing in outside air. Check the grommets and valve cover or oil pan gasket for leaks.
  15. If the vacuum gauge registers a pressure or the vacuum gauge is pushed out of the dipstick tube, check for the correct PCV valve or bleed orifice, a plugged hose or excessive engine blow-by.


See Figure 10

When replacing a PCV valve you MUST use the correct valve. Many valves look alike on the outside, but have different mechanical values. Putting an incorrect PCV valve on a vehicle can cause a great deal of driveability problems. The engine computer assumes the valve is the correct one and may over adjust ignition timing or fuel mixture.

Removal and installation procedures of the PCV or CV valve is located in General Information & Maintenance of this repair guide.

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Fig. Fig. 10: On most vehicles, the PCV valve is mounted in the intake manifold