crankcase pressure

[mountainman]

When I remove the oil filler cap with the engine running the idle increases. Is this normal or an indication of something maybe a bad aos?

[ramo80]

I'm pretty sure if you remove the oil Cap the idle changes. if you find that there is a massive vacuum when you put your on it then its probably a bad AOS.

[JFP in PA]

Normal crankcase vacuum level is about 5 inches, when the AOS fails it jumps near 20, which makes getting the cap off a chore.

[mountainman]

There doesn't seem to be a really strong vacuum when I cover the filler opening, but there is a definite difference in the idle when I open the cap.

[jb92563]

Originally Posted by mountainman

There doesn't seem to be a really strong vacuum when I cover the filler opening, but there is a definite difference in the idle when I open the cap.

That would be normal with any car that uses vacuum to influence various things with the engine.

The computer uses vacuum to gage throttle response, idle etc.

By opening the cap you are breaking the normal vacuum and things will change.