Revs don't change when I remove oil filler cap?
 

Hi,
I've read that the revs should change when removing the oil filler cap with the engine running but mine doesn't.
Is it something I should worry about?
It's a 1977 Carrera 3.0 with about 115000 miles.

Thanks
Bill

Check for vacuum leaks.

Check gasket on cap, too.

Mine doesn't change either. Its the same model as yours.

+ Mine has been fully rebuilt.

Does it drive well? ie: doesn't stall or anything?

Wasserkuhl,

Perhaps it's the Aussie air pressure!

The engine does have a couple of small issues....
Low idle from cold start sub 1000 rpm and high idle when warm say 1050 to 1150 rpm.
Occasionally the revs are slow to fall back to idle when warm and it will fall slowly from approx 2000 rpm to 1050 rpm when I stop.
The car makes good power and runs smoothly.

I've only had the car a few months and recently changed plugs, rotor & cap and think the slow rpm at cold started then but am not certain.
Thanks for all input.
Bill

I once had this issue and couldn't find a vacuum leak. It turned out my mixture was set way rich.

There is a cut-off on the CIS cars where before they don't and after they do.

I think the last year for not having idle drop is 77.

Not absolutely sure but I remember a thread a while ago. I will see if I can find it.

Is it possible it could be as simple as the seal on the oil cap? I have no experience as I just bought my 3.2 in March of this year, just a thought of something inexpensive to try out before you launch into it.

There is no change in idle on the cars before 1978. See around post #8 and after.

http://forums.pelicanparts.com/porsche-911-technical-forum/559701-what-should-happen-when-removing-oil-cap.html

Hi,
I've been searching and reading posts and it seems that vacuum leaks are the sensible place to start looking as well as buying a CIS fuel pressure tester from our host.
I have also found a couple of threads with symptoms very close to mine.
I have a compressor so will look at doing the soapy water positive pressure test for a start.
I am completely new at having a go at the CIS but game to learn.
Besides I'd rather learn how to do it myself than pay someone to fix it and not know what they did.
I've also read that you should have an organised plan when trouble shooting the CIS is their an order anyone can suggest that I do things?
Cheers
Bill

That is correct, Not all CIS cars will experience the idle drop with the oil filler cap removed,
A simple way to check is to verify the routing of the breather hose coming off the oil filler neck, If it goes to the right side of the air filter housing just below the air filter element it will see no change with the cap off,

Later cars have this hose connected above and behind the air flow meter so you create a air leak in the fuel metering when you remove the cap,

Cheers, Lou

Aerodyn930,
I just checked and I have a rubber hose from the filler neck to the right side of the air box housing as you describe. However, I also have another hose (it has a crosshatch finish) that runs from just behind the filler neck and dissapears towards the back of the engine!
So I'm assuming now that the revs DO NOT CHANGE when I remove the oil filler cap on my car as the breather goes to a non vacuum area ie the air filter. What does the second hose go to?
How to now proceed with minor cold start and warm rev issues?
Bill

Lou,

Is the line you're referring to the ~1/2 inch braided cloth hose? Mine is just sitting disconnected in the engine compartment...

Bigel,
The second hose I described as cross hatched you better describe as cloth braided is about 1 inch diameter the same size approx as the rubber breather to the airbox. I'm assuming it's connected to something at the back! It's almost 1 am here so better not go down to the garage again tonight to take a picture or I might wake the wife!
Anyone know what that hose goes to?
Bill

...I don't think the hose I described is 1"...maybe 5/8"...

Steve , The smaller cloth braided line in that area is probably the fuel vapor purge line , Bill, The hose you are describing is the crankcase breather hose, ( goes from top of crankcase vent fitting to oil tank,
As far as CIS diagnostics , Start with a clean fuel filter check your pressures,and verify that you have no air leaks in the induction system,

Cheers, Lou

What's the condition of your gas cap seal? Often overlooked.

Lou,
I have a new fuel filter sitting on the shelf, good advice will start there.
I intend to order CIS pressure gauge from host as I think I'm going to need it.
After fuel filter will start looking at vacuum leaks.
Tanks for your help.
Bill