944meister

I have a 91' 944 S2 that stumbles around a few hundred rpm if I try starting it at a warm/normal operating temperature, and it will continue to stumble around until it dies or if I shut it off. If started when cold, it will fire and run just fine, at most by the second attempt. I had been chasing this problem around all summer and at this point its just getting really old . It got so bad by the end of summer that I just couldn't drive it anywhere. I've changed out many items including the usual culprits, like the dme temp sensor, ignition control module, afm connector, and dme temp sensor connector. It's also got a newly rebuilt afm and dme after it was throwing an afm fault code, which has since gone away. It also has an almost entirely new fuel system, for other less important reasons. The dme relay was also replaced a few years ago, but I tried the spare I had laying around with no change in results. Was thinking that I would do a leakdown test next, but I wouldn't have time for that most likely until Thanksgiving, as I attend university. Leaning towards just selling it and getting something more practical, if the problem isn't anything else that's quite obvious. I'm more than a capable wrench and have been crawling under these cars for several years. Any thoughts on what could be a probable cause????

Justin S2

Try unplugging the isv. If it runs better then the valve is stuck.

F18Rep

I had a pinched wire to the ICS (idle control servo - or whatever its called) once that would make an 944 NA act like that. It would start cold fine but as it warmed, it seemed to get richer and richer until it just died. Naturally this happened on the coldest day of winter and 10 miles from home. I got a lift but the next day limped home by driving for about 3 miles and then having to stop and let it cool again for 30 minutes....Bruce

944meister

Seems a little improbable, as it idles just fine after a cold start and normal running temp. It just won't turn over properly and idle upon restart, when warm. I gave it a shot anyway and it idled horribly until I plugged the isv back in. I don't think it would be different if I let it warm up and tried unplugging to see if it changes, but I could give it a shot.

I have a wideband O2 sensor hooked up and it is running fairly normal richness when cold and stoich when warm.

944meister

Actually, the more I think about it, it could just be the isv sticking. Assuming the valve closes whenever the car shuts off, but is always open some amount when the car is under constant running conditions? and for some reason the valve would just be sticking much more easily after it heats up with the engine. I'm not a hundred percent sure on how the isv operates, but if someone could enlighten me on how it operates and whether this hypothesis is plausible, it would be much appreciated. I have another spare isv sitting around that I could cleanup and test with.

gtroth

At this point run through the DME Diagnosis and Trouble Shooting Guide for S/S2 in the FSM and measure everything.

On the 16V cars the ISV is dynamic- opens and closed automatically and incrementally to hold idle at 850rpm. So if it ever idles right, it's probably not completely seized. If it's a little sticky, the idle might drop too low when you come off throttle.

For hot start issues, I'd first measure
1) fuel pressure and fuel pressure leak down (if car is warm, DME assumes there is fuel pressure so does no cold enrichment). Also, if there is leakdown and it's from a leaky injector, one of your cylinders will havE eexcess fuel in it and run real rich in start up.

2) temp sensor. If the DME thinks the engine is cold, it will run cold enrichment when it's not needed, so might run overly rich.

But again, measure, measure, measure.

944meister

Thanks to you few for the suggestions. I've come to the decision to just go ahead and try to sell the car, along with all the spare motor and suspension parts I have accumulated. I simply just don't have the time for it anymore.

ralaisa

I would check the AFM. Look at the rheostat trace. You will have to remove one of the side covers.

944meister

I just replaced the afm about two months ago with a freshly rebuilt/recalibrated afm from vertex auto

gtroth

Sounds right for where you are - you don't need this extra load while in school, and it's going to need some time.

There will be others down the road.

944hal

Before you sell. My S2 started idling funny, galloping. It also would miss under load. Finally, between a shift from 2nd to 3rd, it quit. It turned out to be the plug on the reference sensor. It dried out, was corroded and basically fell apart. I guess you can't expect much else from a 25 year old car. Anyway, I replaced the sensor (with one from a BMW), it started right up, idled fine and runs to redline without any hesitation.

944meister

The reference sensor plug was one of the things I did inspect and it was brand new about two years ago, but thanks for the tip. Its probably something simple like the isv that's causing the issue. Though, It was new from several years ago and I've already swapped out the other expensive items that would have been a related cause. Very few things on this car I haven't replaced, but it shouldn't be too hard for someone else to figure out. I just don't have the time ATM, so it would have to sit through winter.
I'll be posting a for sale thread later this week, when I get the time. I'm thinking 3.5k-4k should be reasonable for its shape. Its located near the Seattle area. I'd prefer it go to a rennlister, or someone who is a DIY type, as there are a few things to sort out.

944meister

Swapped out the ISV with a known good unit and that didn't fix the problem. Not sure what else to start checking, so I'll just do some testing with Clarks Garage as reference. Still unable restart the car after warm-up, but ran strong otherwise.

944meister

Just did a few tests with the millimeter and the throttle position switch and speed sensor (the one that reads the flywheel) check out fine. However, upon startup I only see a few small tachometer bounces that lie below the first white rpm notch (<400rpm). I'm wondering how much action the tachometer needle should be seeing under starter engagement????
Clarks garage reads that the dme cuts the fuel pump when rpm drops below 300. Could it be that the speed sensor is improperly gapped and causing the pump to cut out, when engine is warm?