Idle speed slows when removing oil cap

[maxman]

When I remove my oil cap to check the oil (car running, motor warm , the motor slows down to a little bit rougher idle. The instant I put the cap back on, it idles right back up.

Normal?

~Max

[Porschedr.]

Depending on how bad it is, it's normal.
Also possible vacuum leak.

[nineball]

same thing happens on my perfectly running SC.

[maxman]

Could have something to do with my slightly poor cold idle? When the car is stone cold, it takes a few minutes before it "finds it's idle". After first starting, It will rev, then slow down to practically stopped, rev again and repeat, gradually smoothing out to a smooth idle after a couple of minutes. Once it's warm, it idles great (when the oil cap is on

[Porschedr.]

I don't think so, honestly. You're inducing a vacuum leak when it's removed. It is normal to screw with the idle when removed.

[brads911sc]

Normal

[equality72521]

Check valve is missing in vent tube to oil tank.

[maxman]

equality72521, Can you provide a little more detail? I'm not familiar. Sorry to be so unaware.

[brads911sc]

not true. mine always idled lower with cap removed... and I had check valve installed. Actually if it doesnt idle lower it means you have a vacuum leak somewhere. idling lower is normal. Unless its stalling out, there is no real issue.

Originally Posted by equality72521

Check valve is missing in vent tube to oil tank.

[equality72521]

The hose connecting the crankcase to the oil reservoir is supposed to have a check valve so you don't introduce a vacuum leak when removing the oil cap. These end up missing so you end up with idle issues when the cap is removed.

[longhornchris04]

small drop - normal... should be a non-issue if your idle is set right.

large drop or stall - missing the check valve

no drop - you have a bigger vac leak somewhere in the system and the induced vac leak doesn't change anything

[maxman]

Thanks longhornchris04.

I think I'm definitely in the small drop category. Sounds like its nothing to worry about.

[crustychief]

Small drop normal.

[NewfieMZ3]

normal......don't panic

[maxman]

Not panicking, just curious

Thanks for all the responses. Very helpful.

[dshepp806]

small drop=normal.......same here.

Doyle

[mikesride]

Uuugh I may have an issue then.......mine speeds up and the small amount of hunting it sometimes does goes right away. Vac. issue of some sort maybe?

Mike B

[longhornchris04]

Mike... I'd start looking for vac leaks...

On CIS, hunting is a sign of over-rich. Then again, a vac leak should add more, not less air, so it may be something else.

[Scott Douglas]

In Mike's case it makes perfect sense if he has an over-rich condition. It causes the idle to hunt but when the oil cap is removed it goes lean and the idle comes up and settles down.
I'd say he something else causing the problem. Don't know what it may be but I'd bet it's not a vacuum leak.

[Hodgey]

Originally Posted by crustychief

small drop normal.

+1