mpeters951

Per title line;

In my '90 C2 (55k miles) I've now been revisited with the hunting idle and under load acceleration hesitation issues I'd experienced last year/summer. At that time I took the car in to a Porsche only shop that I trust unconditionally. The owner/long-time master Porsche wrench and I went through, as I recall, just about everything. Pulled fault codes, replaced cyl head temp sensor, swapped in/out Motronic with a known good box, thoroughly cleaned the air intake/metering electronics and mechanicals - all to no avail. Same intermittent/hunting/crappy idle, terrible hesitation under load, seemingly misfiring and/or running on less than 6 cylinders. We've verified the TPS as clean and working just as they should. All other bits and pieces hanging atop the intake manifold verified as good/sound/acting normally. All up, in addition to the steps above, we wound up cleaning the ISV and then the car ran flawlessly for last season.

Return of symptoms very recently. Checked all that I could, referring to Adrian's book, Rennlist posts, et al. No change to the very intermittent coming and going of the full cast of symptoms as above. Carefully and completely re-cleaned the ISV and its' electrical connection, checked the battery connections, car is running normally for a few days only for symptoms to return without rhyme or reason - jeez WTF ?

The thing that bothers is that neither the shop, this thread or others I've read have seemed to have gotten the root cause. One bizarre element in this is that while I was cruising merrily along one morning, car behaving well, I hit a good bump that jolted the rear suspension a bit and instantly the rough running and hesitation symptoms returned. I can recall precisely that such a jolt from a bump to the rear suspension has immediately preceded the onset of symptoms at least twice. I've gone back and rechecked the ISV and it is perfectly clean. My shop suspects that the off idle hesitation should not track back to the ISV.

Porsche shop now having heard of return of same symptoms as before is asking that I pull the Motronic box, which I'll gladly do, in order for them to verify all the solder traces and fully test the box. At their suggestion I'll get the panzer warmed up and then pull the seat and tap/bang on the Motronic a bit while idling and see if there is any change in idle characteristics.

Any further thoughts and any who have experienced this seemingly common set of symptoms, I'd appreciate any and all opinions and experiences in the interest of tracking to a permanent fix.

Mark

dfinnegan

There's a TSB that may interest you:
Engine - Erratic Idle, Loss of Power, or Stalling

C4Russ

There is nothing more difficult to diagnose then a intermittant problem. If this is, as you alluded, related to a trace or circuit board issue, you'll have trouble trying to find the trouble even with a known good box if jarring can fix or initiate the symptoms.

You could start with basics like the DME relay, throw it out and get a new one.

The only thing I recommend is that you do only 1 change at a time. Or go wholesale and change everything but that is not recommended as it may miss the problem and just cost a whole bunch of money.