engine idle while oil cap removed question

[Porsche-poor]

Just finished changing the oil and went through the fill up drive and check procedure. I opened up the oil tank there was only a subtle change in rpm. I was under the impression I would hear a big change. Let me guess a vacuum leak some place else?

[mca]

My RPMs drop pretty significantly when I remove the cap.

I have never looked at tach to see how much it drops but it is a very noticeable change.

It may be best for you to see someone else's so that you have something for comparison.

[Car Guru]

I'm curious too, I checked mine after the last thread on this topic. My RPM barely changes, only a keen ear would tell. Is it suppose to change?

-Ryan

[Bill Verburg]

Yes, idle rpm will change. The oil tank vents into the intake manifold. when the cap is removed it allows more unmetered air to enter the engine. There is a restrictor that reduces the amount of air other wise it would just die.

[Porsche-poor]

So as long as there is a change its ok even if its just slight?

[300cd300sdl911]

Just jumping in ......on both our 911's we have a significant drop in rpm (or at least sounds like it) when the oil filler cap is removed.

[Formerly Steve Wilkinson]

What year 911s? I think that makes a big difference, doesn't it?

[300cd300sdl911]

Both of our's are 1985 - one ROW the other US spec.

[Porsche-poor]

so does this mean that my engine idle is compensated for a vacuum leak else where and it makes not diff if the oil cap is off?