A crankcase ventilation system is used to consume crankcase vapors in the combustion process instead of venting them to atmosphere. Fresh air from the throttle body is supplied to the crankcase, mixed with blow by gases and then passed through a PCV valve into the intake manifold.


The primary control is through the positive crankcase ventilation (PCV) valve which meters the flow at a rate depending on inlet vacuum. To maintain idle quality, the PCV valve restricts the flow when inlet vacuum is high. If abnormal operating conditions arise, the system is designed to allow excessive amounts of blow by gases to back flow through the crankcase vent into the throttle body to be consumed by normal combustion.

A plugged valve may cause the following conditions:

• Rough idle

• Stalling or slow idle speed

• Oil leaks

• Sludge in engine

A leaking valve would cause:

• Rough idle

• Stalling

• High idle speed

If an engine is idling rough, check for a clogged PCV valve. Replace if required. Use the following procedure:

•Remove PCV valve from intake manifold.
•Shake valve and listen for the rattle of needle inside the valve.
•If valve does not rattle, replace valve.
With this system, any blow-by in excess of the system capacity (from a badly worn engine, sustained heavy load, etc.) is exhausted into the intake manifold and is drawn into the engine.

Proper operation of the crankcase ventilation system is dependent upon a sealed engine. If oil sludging or dilution is noted, and the crankcase ventilation system is functioning properly, check engine for possible cause and correct to ensure that system will function as intended.

More Auto Repair Help

Please login to comment

Comments (2)
   ---Subscribe To Your Comment   

  • Guest - Amy

    My 2002 cavalier just started bogging upon fast acceleration. (Like turning on to a highway from a complete stop.)
    I could hear it!
    Auto transmission shifted into 1st and 2nd gears, I believe normally, I was preoccupied with listen go the bogging noice. But 3rd gear, the RPM's went higher then normal. I actually let off the accelerator alil so it would shift properly.
    I've owned this car for 10 years. 174,000 miles. 2.2L what's happening, or causing this?

  • Guest - AMY

    2002 chevy cavalier, automatic transmission, & 2.2 liter engine. 173000 miles. Started bogging upon fast accelerating and the RPM's went up past 3 before shifting into its 3 gear. Normal RPM's 1st and 2 gears shifting. But it was bogging terribly. I've owned this car for 10 yrs. Have kept up with routine maintenance.
    What is going on now?