Porsche Enthusiasts Club Forum

I have a strange hot start issue with my 944S2. If I start the car from cold it starts and runs ok throughout warm-up. The idle can be a bit lumpy at times, and it seems to be running a bit rich when warm, but it will run and drive.

If I start the car when it's hot, it will start fairly easily (cranks over a little longer than when cold), and will idle but quite roughly, and you can smell it's running really rich. When I then try to rev it, the revs will rise briefly but then start to bog down and the revs will gradually drop until it stalls, usually accompanied by some pops and bangs. If I release the throttle when it starts to bog down it will go back to rough idle. When doing this I can smell it's running incredibly rich.

So it seems like when I do a hot start, the car is fuelling as if it were a cold start. It's as if switching the engine off causes the engine to lose all it's temperature readings. The only way I've found to drive it after a hot start is basically to let it idle for about 10 minutes without revving it, and then it seems to lean out enough that it will drive reasonably normally again.

I have two DME relays, so I've tried swapping between them, but it makes little difference to the problem. I've also replaced the coolant temperature sensor near the front of the crankcase with a new Bosch one, and that didn't make any difference either. There's a temperature sensor in the underside of the intake manifold which I have a spare used one of from my old engine so I'm going to try swapping that over to see if it makes any difference. However I cannot find any mention of that temp sensor in the PET diagrams, and there is no part number displayed on the sensor itself.

So I have 2 questions:
- what is the Porsche part number for the temp sensor in the intake manifold / does it appear anywhere in the PET diagrams?
- what else could be causing it to over-fuel so much after hot start?

Sorry for the essay!

Fair chance your idle control valve is stuck, or there is a vacuum leak in that area. On the S2 you should just be able to get the icv out an in without removing the intake manifold. There is a basic test procedure on ClarksGarage.com to check it's working, if it's stuck you might be able to free it with wd40.

Also check your TPS for correct operation

Thanks for the suggestions.

Having now read up on what the ICV does and how it works, that definitely seems like a likely culprit and a good place to start, so I will remove, clean and test it (and possibly replace it), and see what difference that makes.

I'll read up on how to check TPS functionality and then check that as well.

I had similar symptoms though not as severe - when warm it would die when off the gas, slowing for lights etc and run rough when restarted. Also struggled to pass the emissions tests at MoT.

I did the following:
Changed DME temp sensor
Adjusted and then replaced TPS
Cleaned and tested ICV
Replaced AFM with a remanufactured unit.
Fixed some potentially dodgy wiring to the hall sensor

None of these fixed it!

Then as part of a clutch change, I had a new flywheel crank and 02 sensors fitted.

Ran perfectly after that!

I believe it was the crank sensor.

Yep crank sensor is favourite if the isv is ok. On the 993 it would run for 15 mins then die, after cooling down it would fire up then die again. On S2 it would start to misfire when warmed up.

I replaced the speed/reference sensor last year which is what I presume you mean by the crank sensor, so I don’t think that’ll be to blame.

I’ve removed the ICV, cleaned it and tested and it seemed to be working ok. However, I still have the intake manifold from my old engine which happens to have the ICV still attached, so I removed that one too and tested it for comparison.

Comparing the two ICVs at the inlet port, they’re both open when no voltage is applied, and both close when I apply 12v. However, looking more closely, the ICV from the car is only about 80% closed (it seems to close fully then bounce open slightly when voltage is applied) whereas the ICV from my old engine closes fully when voltage is applied. This means the ICV on my engine is slightly open all the time, which could be the cause of it running rich.

Try reseating the O2 sensor connection. Sometimes you get a bit of corrosion in there and a clean/reseat of the connections will help...

We had three 944 S2's on the Le Mans trip a couple of weeks ago and all of them had this same hot problem with a rough idle initially after a hot start but once running were all fine.... as all of them reacted the same at the time (and were fine in the cool air of Dover) we put it down to 35 degree heat...

Will be watching your follow up on this with interest though