oil cap vacuum

[Ederd]

Is there supposed to be any vacuum on the oil cap while the engine is running. Currently my car is at :

950 RPM with the cap on
850 RPM with the cap off
900 RPM with the dipstick out.

Is this normal or not?

[Gawernator]

Well the Oil fill is connected to the AOS which uses vacuum so I would say yes. I just replaced my Oil cap seal in my quest for fixing AOS issues and vacuum leaks

[Ederd]

I was wondering bc on my other two vehicles there was no difference in idle when I took their oil caps off for a comparison. Different engines, 2.5 KV6 and a 5.2 V8, but I was seeing if the principles were the same. Learning 3 engines at once is a little confusing sometimes.

[matty89]

U can Aleays spray intake cleaner around the places I think there is a vacuum leak to confirme it!

I was wondering bc on my other two vehicles there was no difference in idle when I took their oil caps off for a comparison. Different engines, 2.5 KV6 and a 5.2 V8, but I was seeing if the principles were the same. Learning 3 engines at once is a little confusing sometimes.

[16vCorey]

Some engines do, some engines don't. As for the 944, I'm not sure. Just for reference, a BMW M20 does pull vacuum from the valve cover, and runs like total ass with the oil cap off. I found that out the hard way last time I changed my oil in a hurry!

[Gawernator]

Again I will say yes. The oil fill is part of the air oil separator. Whatever goes in there will go to the intake. And it works off vacuum. It only makes sense you would be leaking vacuum with the cap off.