Ok, so I just bought me a 1991 Vert like 2 days ago and noticed that the oil was low and put some in. I stopped to check it after driving a down the street. I noticed as I took out the dipstick that I heard a *psh* like it was releasing vacuum. It was strange, never happened to my 87. So I put the dipstick back in, waited about 10 seconds, took it out again to hear the same *psh* Tried to take the oil cap off and almost tore my hand up getting it off. I knew I didn't tighten it up that much.

So I've come down to 2 reasons why I have that much vacuum build up in there. I think either the PCV valve is stuck open or the pcv valve regulator(forgot what it is called sorry) is constantly giving the pcv valve vacuum. I also have a oil leak somewhere. Haven't had a chance to get up under there yet.

tl;dr High vacuum in crankcase, why?

 

Normal...The crankcase is kept under vacuum by the purge valve.

 

Ok, I did not know that, thank you. Just was wondering cuz I've never seen a 7 that makes the *psh* sound and makes the oil cap unscrewable. But I trust you so

 

maybe a difference in the S4/S5 but S5 oil caps have always been a bitch to remove, for me atleast.

Next time, use a rag I cut up my hand before, not fun.