Poor Running

[Guest]

Have just acquired the 911 and a fault has rapidly appeared. On start-up the engine revs jump up and down, nearly stalling. After driving the problem goes away.
I assume that this relates to an automatic choke but any suggestions would be welcome.

 

[engleflick]

Hi Clifford,

Mine does this too, I believe it is the cold start valve (its called somthing like that, but I can't remember) when starting from cold first thing (especially when not having used the car for a while) the engine 'hunts' for revs - I have had my car for 4 years and it is on my to do list, but I haven't got to it yet!!!

If you start the car without touching the accelerator, then once it starts to 'hunt' gently push the accelerator so it is reving about 1200-1500 rpm (mimicing what the cold start valve does) and after a couple of minutes you should be able release the accelerator and the car should idle correctly - not ideal but a temporary solution []

 

[bones]

It's more likely to be a vacuum leak Dan, when I refurbed the injection on my SC it hasn't hunted for more than a few seconds since, I replaced all the hoses including the six big that are connect to the air box. Your method for overcoming the hunting is a good trick though.

 

[Guest]

Hi Cliff,
If all checks out OK with the vacuum as suggested by bones, I recommend you check out the auxiliary air bypass valve. This valve should be open on cold start to allow more air into the engine to raise idle speed during the warm up stage, as the engine warms up the valve closes by heating a bimetal strip. It is the same as putting your foot on the throttle during warm up i.e. letting more air into the engine. If this valve is shut during warm up the engine will hunt and be very lumpy, due to rich mixture and if the engine does ticks over it will be at about 500RPM and hunting.
This valve is on top of the engine on the right hand side, it has a large pipe (approx 20mm dia) connected to the inlet and one to the outlet of the valve. To check the valve is working correctly you will need to take it off, it is secured by 2 small socket head cap screws and attached to it is an electrical plug. With the valve out of the car and cold it should be open, therefore you should be able to blow through it. If you can then the valve is correct for cold start , if you can't then this valve is most likely faulty.
My engine had the same cold start problem as yours and this valve was faulty on my engine.After changing the valve for a new one the engine starts and run at about 1100 rpm rising to 1500rpm then over the next 3 to 4 minutes the revs fall as the valve slowely closes to tick over @950rpm.
Hope this helps
John

 

[Guest]

Thanks for all three excellant replies. I will investigate.

 

[Guest]

Hi Phil,
I bought the new valve from the Porscheshop phone 0870 0055 911.
@ £111.39. I decided not to bother with a second hand unit that could be 20 years old or more.
Cheers
John