dugsud

My 83 SC runs superb in every way but I was just wondering how much the revs should drop when the oil filler cap is removed to check the oil?
I understand a drop is normal but mine drops from about 800 RPM to around 400.
It can even stall if the cap is left off for some time!

Any thoughts, opinions?

Thanks

wwest

My 78 SC targa does the very same thing but I generally don't run it very long with the oil cap removed. My presumption is that depressurizing the crankcase has some effect on the PCV system, maybe letting air into the intake system that bypasses the mixture control.

bones

That sounds normal Doug although you could increase your general tickover speed to 850/900 rpm.

dugsud

Thanks, that's one more 911 0wnership worry I can tick off

spaceman

Yup, the rpms drop. Note that the correct way to check the oil level is with the car warmed up, running, and parked on a level surface.

spyderman

um, yea, i had this problem also, i even went as far as to check the CO with taking the cap off and on. with the cap off the CO dropped about three percent. this is caused by EXCESSIIVE blowby vapors being burned, the car will actually run on these vapors, when you take the cap off, all the vapors go out into the atmosphere, and the car leans out, and will stall. Do a compression and leakdown check, and see what kind of leakage your getting. its probably not the best, and almost time for some new pistons and cylinders.

-Ken

bones

I've come across many 911s and all I've seen with CIS based induction drop revs when the oil cap is removed it has nothing at all to do with worn parts and that's not an opinion it's based on years of ownership and being around mechanics and other owners. The 911 dry sump oil system is pressurised; when you remove the oil filler cap the vacuum pressure is effectively dropped and as the CIS system is a part of the this whole cycle so revs are affected. The importance of maintaining this vacuum is referred to in my 1978SC owners handbook, when in the section telling the owner how to check the oil it reads '....a loose cap will cause a rough engine idle....' well if a loose filler cap will cause a rough idle guess what happens when you remove it!

Jens

Completely normal.

DonMo

That is completely normal for a Pcar. There is a vacuum to the oil resevoir. The overflow hose has a small restrictor in it that keeps the vacuum from tanking all the way but the idle definetely drops. If the restrictor is gone, the car will actually stall out. You can get around it by using a set of vise crips or a clamp and pinch the hose closed. Don't worry about it.