Idleing Problems

[hecatonacles]

I am new to the forum and didn't turn up anything quite right in my search. I recently got a 1986 automatic 928S. This is my first Porsche and I am a little intimidated by it as the parts for the thing seem to be made in the black forest by elves and for the 928 they seem to be mostly dead or in hiding.
Over the winter my car developed a problem keeping a steady idle. The idle kind of pulses. I thought at first it was a system to warm the engine and it just didn't like the cold when I was in Kentucky with the car, but I am now in Arizona and still have the same problem. The problem is not regular either sometimes it is their and sometimes it isn't. I can drive the car turn it off get gas and then start it and the problem is back. Oh this also kills the engine if the car is in gear and not under throttle. I have run fuel injector cleaner through the system so far with no help. I had a problem that was similar to this in an Audi I had once and it was fixed by replacing an idler relay. I don't even know if I have such a thing in this vehicle. Any advice would be appreciated.

[bwmac]

Welcome Hecta, you got a good car. I am new to as I just bought my 89 last fall. search out the forums for 928s there are a few. This one is awesome and the people here are very helpful.
here is a link to a thread with lots of good sites
928 specific tech website?

[Danglerb]

Long list of maybe stuff, and could be more than a few if not all of them;

Vacuum leak.
Old MAF.
Bad ISV idle stabilizer valve.
Bad crank sensor.
Bad grounds.
Bad temp sensor (I think yours has 3 or 4).
Bad throttle position sensor.

[JhwShark]

Hec',
Welcome. You will find this a great place to get help and learn how to help yourself. I do not have your model but others will chime in. You issue can have a few paths before a resolution...grounds, Central Electronics (CE) board connections fuses and relays, vacuum lines, fuel system, and any related component...

I recommend that you find a WSM (workshop manual) and PET (parts diagram) so that you can orientate yourself to the car, its parts, where they are, what they are called and their TLS's (three letter acronyms). They are available on-line, I just don't have the links. The guy's and gals (I hope there are a few) will refer to these at times as well as the wiring/current flow diagrams.

Jon

[jstobo]

Jumper the test connection at the front of the motor per the manual to adjust idle. Jumpers go on the 1 o'clock and 6 o'clock connectors. This shuts off the Idle Stabilizer Valve. IF it smooths out then I would replace ISV. If it stays the same you probably have a vacuum leak or your air bypass on the throttle body is carboned up and not letting air bypaa the throttle plate. This screw valve on the throttle body is how you set your idle. You can unscrew it completely and spray Chemtool b-12 to de-carbone but then you have to reset your idle.