yet another idle problem!

[FransB85]

after replacing the brake booster last autumn I had finally hunted down the cause of my previous idle problem. now however, I have a new one! (seems to be part of the 944 experience)

On cold starts the engine runs very rough and will hardly/not keep running (unless you help it with a bit of throttle). the funny thing is it only does so for the first minute, after which it settles to a nice and steady idle, no problem at all! It runs nicely through the rev range on both petrol and LPG, no loss of power or whatever.

I have recently replaced the fuel pump because it was leaking pretty badly and I hoped this would also solve this idle issue, but sadly it didn't.

Parts I'm fairly certain aren't the cause: ICV (new) FPR (new) brake booster (new) fuel pump (new) alternator (new) battery (new) rotor and cap (fairly new) HT leads (new). Also, I cleaned and resealed the injectors and replaced a leaky one. all this was done quite a while before the problem first occurred.

to my logic and experience a substantial vacuum leak would cause bad idle at any time, not just the first minute after starting up.

My main suspect in this case is the DME temp sensor. Anyone else have a better idea? By the way, my car is a no-cat euro model, so no O2 sensor.

your input is very welcome

[guru944]

FransB85:

My first thoughts were the O2 but that doesn't apply to you. I would say next sensors are DME Temp and AFM Temp are next. What gap and plugs are you running? I found Denso/NGK Iridiums work pretty well gapped at .032. I just did my HV fuel pump and wow! My car no longer stalls after it gets warm or 10 min....
New DME Temperature goes in later or tomorrow..... Then a better retune.

[FransB85]

I believe I run Bosch plugs, didn't gap them to spec just put them in. I lean towards the temp sensors because of the temporary cold start associated nature of the issue. Afm temp is not replaceable I guess?

[uvachief]

Cross reference sensors maybe? I also hate to ask this but have you had a compression test done? If not, get one to see if you have compression off in a one or more cylinders. Let us know what the readings are. Bad cross reference sensors or leaking/worn rings normally prevents it from starting at all.

Also, was the problem worse or better before you cleaned the fuel injectors?

[FransB85]

problem occurred months after all that. just out of the blue really. I have no reason to believe my rings are worn, hardly uses any oil considering how much I enjoy the entire power band. Also, it;s almost impossible to turn the engine over by hand when it's cold because the compression is so high. So I'm not worried about that.

As I said, the engine idles fine after about 30 sec-1 minute, no problem. So it really seems like some digital malfunction.

However, the question about the spark plugs has reminded me that I'm probably a bit overdue for a plug change, it's been 20.000 hard miles, mostly on LPG so they're probably not that fresh. Still, I can't imagine they'd just make it run weird for the first minute. Ref and speed sensors would to my knowledge stop it from running at all, so I kind of ruled them out.

[JonA]

I have the same problem, I believe the idle control valve is not kicking up the initail rpm's. Took it out and cleaned and things were good for a few weeks but now it is idling rough again like it was before Temp sensor is good, checked with multimeter against resistance table.

A quick check is to unplug the ICV and see if it makes any difference. You can do that without taking anything apart so it is relatively painless.

Let us know how you make out. Good luck.

Jon

[Gawernator]

Sounds a lot like DME temperature sensor to me. I think unplugging it when cold... I think that defaults it to cold

[petrom3]

just fixed a similar gremlin.
in my case it was bad solder joints on dme relay. as soon as i saw circuit board i knew that was the problem. just pull relay and reflow solder joints, i could see the bad joints with the circular rings around the solder cone. and this was on a newer relay. i now see why you should keep a spare. please check if this is your issue. it matches the symptoms

[FransB85]

Oh yes my dme relay is also new, the upgraded 993 type... I'll give unplugging the temp sensor a try

[Eric_Oz_S2]

URO brand DME relays are rubbish from my own experience. If you have one of those bin it and get an OEM one.

[petrom3]

even worth checking if new 993 version.
mine was a new 993 version and it was bad. the solder joints do not hold up well especially the two joints at top of board. known issue. only takes a few minutes to check, another 10 to reflow solder joints adding a little extra solder. if i bought a new relay myself i would reflow the solder before using

[FransB85]

but wouldn't a bad dme relay cause the exact opposite symptom, where the problems occur as soon as it heats up?

[petrom3]

yes you are right. reread the thread and saw yours was on cold start. mine was on warm start, cold start was fine but some warm starts were stumbling.

[FransB85]

unplugged the DME temp sensor connector (blue plug under throttle body right?) and it didn't help, in that it would not start at all so I replugged it. again, after about 30-45 seconds it ran perfectly and has done so for the rest of the day...

[JonA]

Clarke's Garage has a write up on DME temp sensor testing. You need an ohm meter to check resistance.

Jon

[Chunkerz]

Is the ISV plugged in? How do the wires for it look? Is the ISV getting power? There's a video I saw on YouTube of some guy that deleted his ISV and now he has to hold the throttle open for about a minute on cold starts or else his car runs like ****. Sounds a lot like what you're experiencing, so I'd check out the wiring for the ISV.

[Lapponia]

It´s though to start an engine with oil grade of 20W-50 dino in outside air in Europe at 10-12 C. I´ve noticed this in Sweden the last few weeks with that oil in my 1984, 944NA when running up the engine to full temp. I also changed the so called DME temp sensor late fall and found low rew in the beginning after start. I guess the "Additional Air Valve" is somehow stuck and does not give the extra air for the first minute(s). Engine runs fine after a short while and all day.
To my surprise the new DME temp sensor has eliminated an old bug in that the engine now runs smooth again after a short break (coffe or shopping) without the ever going "sputter" after start. I´ve have had this problem for 5-6 years and it seems this has gone now.
I also checked the resistance in the old DME temp sensor according to Haynes and it was OK but I think it must have been issues in the sensor though as the new one gives the right behaviour. Good luck

[Lapponia]

Oh! Additional Air Valve should be read as Supplementary Air Valve. Sorry.

[FransB85]

Originally Posted by Chunkerz

Is the ISV plugged in? How do the wires for it look? Is the ISV getting power? There's a video I saw on YouTube of some guy that deleted his ISV and now he has to hold the throttle open for about a minute on cold starts or else his car runs like ****. Sounds a lot like what you're experiencing, so I'd check out the wiring for the ISV.

this sounds pretty sensible! thank you, I'll have a look at it

[Guest765]

The iacv is required to make the car idle, period. Would love to know exactly how he deleted his and it didn't run like **** 24/7