GM Lumina/Silhouette/Transport APVs 1990-1999 Repair Guide

Crankcase Ventilation System

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OPERATION



All vehicles are equipped with Positive Crankcase Ventilation (PCV) or Crankcase Ventilation (CV) System to control crankcase blow-by vapors.

A crankcase ventilation system is used to consume crankcase vapors in the combustion process instead of venting them to atmosphere. Fresh, filtered air is supplied to the crankcase, mixed with blow-by gases and then passed through a PCV valve into the intake manifold. The crankcase ventilation system must be operating correctly to provide complete scavenging of the crankcase vapors.

See Figures 1 and 2

Click image to see an enlarged view

Fig. Fig. 1: Schematic of the vapor flow through A PCV valve



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Fig. Fig. 2: Common Crankcase Ventilation Flow 3.8L Engine shown in the Diagram

The PCV valve meters the flow at a rate depending upon the manifold vacuum. To maintain idle quality, the PCV valve restricts the flow when inlet vacuum is high. If abnormal operating conditions occur, excessive amounts of internal exhaust gases back flow through the crankcase vent tube into the air filter to be burned by normal combustion.

COMPONENT TESTING



A plugged valve or hose may cause the following conditions.



Rough idle
 
Stalling or slow idle speed
 
Oil leaks
 
Sludge in engine
 
Oil in air cleaner
 

A leaking valve or hose may cause the following conditions.



Rough idle, possibly a rolling idle
 
Stalling
 
High idle speed
 

See Figure 3

Click image to see an enlarged view

Fig. Fig. 3: Remove and shake the PCV valve; if a rattling noise is heard, the valve is OK

If the engine is idling roughly, a quick check of the PCV valve can be made. While the engine is idling, pull the PCV valve from the valve cover, place your thumb over the end of the PCV valve and check for vacuum. If no vacuum exists, check for a plugged PCV valve, manifold port, collapsed vacuum hose, or deteriorated hoses. Turn the engine OFF, remove the PCV valve and shake it. Listen for the rattle of the check needle inside the valve. If it does not rattle, replace the valve.

The PCV system should be checked at every oil change, and serviced every 30,000 miles (48,000km).

 
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