Porsche 911 SC starts and drives fine but rought idle

[artojma]

I have a MY 1981 ROW 911 SC with a smog pump, no a/c and in totally original condition (f.ex. no pop off valve). It has done about 82 000 kilometers and it has been serviced regularly.

It was running just great but after 12th of May this year it all changed – it start losing it´s idle: first for about a month or so if it was started cold it would idle very roughly about 300 rpm but if it was given a bit of gas it would idle after about 30 seconds OK on it´s own– if it was started warm it would idle just OK right from the beginning. After end of June it has mainly idled about 500 rpm but sometimes idles just normally ie. 950 rpm - if started cold it idles first about 1 800 rpm for a few seconds.

Spark plugs, distributor cap and rotor all look fine, a vacuum test didn´t show any leaks and there is also no gasoline leaks to see and if I pull the oil filler cap off the rpms drop a bit, no cracks on the air box, no back fire, besides the idle it runs just fine – so it always revs and starts fine !

The exhaust analyzer gave these results just a couple of hours before idle problems started:

CO 0.23
HC 10
CO2 12,70
O2 3.71 %

Since the nearest Porsche service is about 1 000 km´s from me I would really appreciate any ideas what could be wrong in my Porsche – even if it would not be better than a guess !

[tirwin]

Let me suggest some reading. Search for "CIS for Dummies" and you will find a thread I started that should help with some understanding of potential problem areas. Since you have a ROW car, there will be some notable differences so be aware of that.

When I first got my '83 US spec I had very similar symptoms. I had a combination of problems. First was poor spark. I changed plugs, plug wires, distributor cap and rotor because it needed to be done. That made a big difference.

The second thing was running the fuel pressures test (write-up on how to do it is in the Dummies thread). Running that test revealed my cold control pressure was too low.

Vac leaks can be hard to track down. I suggest using the method described by Tony (boyt911sc). Link is in the Dummies thread.

Hope that helps to get you started.