Poor idle during warm-up

MAS · MAS's Garage

My 77 911S runs and idles well when warm. But on cold start-up, it idles OK (at about 1,200 rpm) for about a minute, then the idle suddenly seems to "switch off"... almost like an automatic choke shutting off. Then the revs drop down to about 500 rpm and then within a few seconds, unless I blip the throttle... it stalls.

Needless to say, on short hops, this makes car a bit of a handful to drive.

Any ideas?

Thanks,

-MAS

porsche_virus · porsche_virus's Garage

Turn idle screw.

MAS · MAS's Garage

OK... but if I adjust the idle (up), then won't it idle too high when the engine is warm?

-MAS

mb911 · mb911's Garage

Yes but if you move it slitly it should solve the problem and have no adverse effects.

Ben

porsche_virus · porsche_virus's Garage

My '78 3.0 did the same thing when I put it in my '74, and I had to adjust the idle WAY up for it to keep running when warm, but it holds strong at 900 rpm now every time. When starting it cold, it runs up to about 1500 for about 30 seconds, then slowly revs down to about 1000 for a few minutes, then to 900 after about 4 minutes. It might change with the climate a little as well. It's been in the 90's here with lots of humidity, and it idles a little high when I'm sitting in traffic, but after coming off the highway it idles fine.

Bill Douglas · 07-02-2003, 07:26 PM

I just give my car a bit of gas after I start it up. It is VERY bad for a cold engine to idle at about 900. Ideally, with our engines, it should be at about 1,800 or 2,000 for the first few minutes at least. This is due to the fact that idling is tough on the valves and cams and what is tough on valves and cams is hell on them when cold. The cam lobe shape is optimised for revs 3,0o0 and above.

MAS · MAS's Garage

OK... on my old 87 924S, the idle adjustment was easy enough to see. But on my 77 911S, I can't easily see where it is, or how to adjust it. Any pointers?

Thanks in advance,

-MAS

mb911 · mb911's Garage

It should be easy to find you may have to remove the heater fan assembly next to the air box. There are 2 screws. One being the air bypass screw ( the one you want to adjust) and 1 that is on the bottom of the throttle plate. The one on the throttle plate is said to not be touched by all my CIS books. The air bypass is the on to move.

Ben

77 911 3.0
75 914-6 2.7

michigan · 07-07-2003, 01:23 AM

I too have a 77 911s. You should not have any problem visualizing the idle adjustment screw on your car. It is large and cqn be turned by your fingers. It sits to the left as you are looking into the engine compartment of the air flow sensor. Follow the fuel linkage as it comes up from the linkage tunnel to the control unit. It is very sensitive to adjustments and, you are right, should be done when the car is fully warmed up. I don't believe that the idle adjustment comes into play until the wur and cold start valve have done their job first. The enrichment adjuster is another story and should only be done with a CO2 meter. My start up idle, cold, is 1800 or 1900 rpms with a gradual decrease down to 950 to 1000 rpms. Good luck. Let us know how you do.