What should happen when removing oil cap ?

[prebordao]

I remember reading sometime ago that when you remove the oil cap, while idling, that revs should drop.
Well mine doesn't... what does it mean ? a vacuum leak somewhere ??

[Oh Haha]

RPMs should drop a few hundred or so.

Yes, a vacuum leak is typically the source if they don't drop.

[Bob Kontak]

Correct. I read this test in a lot of posts where folks are having CIS running issues. If it does not drop the rpm, you may have a real big leak - which can be easier than a real little one(s) to find.

I did see a post from John Walker's Workshop about the brake booster hose cracked under the car - by the transmission - that would be impossible to find from the engine bay.

I found one of my injectors popped out of the intake runner once. I have taken off the CIS and the fuel lines were a little bent so I straightened them on re-install but left too much stress in the #6 line - pop goes the weasel.

Also check if your air bypass adjustment screw is all the way in you definitely have a vacuum leak.

[arbita1]

If you can't find any major air leaks, then check your CO setting.

I had this problem and couldn't find any severe air leaks. I did find that my CO was set extremely rich. Once I brought it into spec, the RPM's would drop with the cap off.

[bklyn]

Would this also apply to an 87 3.2? Because my also does not make any difference with the cap on or off.

[JJ 911SC]

Originally Posted by bklyn

Would this also apply to an 87 3.2? Because my also does not make any difference with the cap on or off.

Yeah mine 86 3.2 does drop a few hundred of RPM. You should hear it when you take the cap off... and will rev up a bit when you put the cap on.

Good Luck

[bklyn]

Well I guess I am going out to the garage, maybe I just cant hear it because it runs great.

[Wyvern]

Depends on the year vehicle ..
My 75 has the breather routed before metered air ... doesn't make any difference with cap on or off.

Your sig says "74 911" so you may be like mine

[JJ 911SC]

Originally Posted by bklyn

Well I guess I am going out to the garage, maybe I just cant hear it because it runs great.

Don't waste money... If it working fine leave it alone and just do the usual air leak fault finding by spraying carburetor cleaner around the hoses.

P.S. If you are Pitch Deaf Tone... Check the RPM before you take the cap off and check the gauge after you take the cap off.

Good luck

[prebordao]

On my 74 there are 2 breather hoses coming from the oil tank. one goes to the airbox right behind the filter (so before metered air), the other goes to somewhere behind the throttle body, I can't really pinpoint where, but it should be after metered air.

[Flat6pac]

The system wasnt closed to affect the idle until 78 model year.
Bruce

[dshepp806]

Originally Posted by bklyn

Would this also apply to an 87 3.2? Because my also does not make any difference with the cap on or off.

hoses' oil restrictor missing?

[Bob Kontak]

Looked at the parts diagram - no restrictor before '78. Bruce is correct.

[s_morrison57]

should it do the same on a 930?

[Bob Kontak]

S_Morrison57 - Link below shows you how to get to the parts diagrams - This may be old news to you.

However, the oil systems look identical in the 911 and 930 through 77 and they look identical from 78 on in both as well. But it changed in 78. Pre78 you have to look at two sets of diagrams. 78 forward they are merged.

The restrictor valve came to play in 1981 - probably in response to the idle dropping too low with the newer system. This info is just from looking at the diagrams - I can't explain the real reason for the closed system as Bruce describes it. I looked at my RoW diagrams and they are pretty much the same as the US ones.

FWIW - I have a friend with an 89 930 and I know the idle drops in his with the cap pulled.