MacinTek

Hello rennlisters!! I have a 1987 944 n/a. It has just under 276K miles on it. Yesterday, I started it up and the idle revved over and over from about 1000 RPM to about 1800 RPM. Today it does the same thing.
Here are some interesting notes from my feeble attempts to solve the issue... Once started, the idle will stabilize IF I disconnect the AFM.. but if I try to open the throttle, it dies. I tested the O2 sensor, it seemed to work alright. I tried deisconnecting the idle stabilizer... no difference. The intake boot is practically new, so it's not worn out. If you have any ideas.. please let me know. Thanks in advance.

Chris_924s

miles - welcome to the board- i suspect bad DME. its worth a chance to re-solder it.

Chris_924s

also- check vacuum lines- my PS vacuum line is now plugged, but i get an 800 rpm idle, then 1100 where it should be.

Wormhole

You may want to test the throttle position sensor. They have a tendency to go bad on these cars and typically cause the problem you describe. I believe the hans manual has specs you can test against.

MacinTek

Hmm... would it run at all with a bad DME? I thoughjt about that but assumed that it wouldn't run at all if the DME were fried.

MacinTek

Am I correct in assuming that the 951 TPS is totally different from the N/A unit? If this is so, does your recommendation still have merit?

Wormhole

All TPS serve the same function. The terminals and ohm reading will be different for each application, but the problems associated when they fail are the same.

7thStranger

You said you disconnected the idle stabilizer and no difference. Did you do this before starting the car?

If you disconnect it when the engine is cold and start the car you should notice that it idles a bit higer. If it does not then it is sticking.

If you disconnect the AFM the car should not start at all.

Another thought. Is the engine breathing ok. Good intake and outake. If you give it gas and dies. Is your Cat ok?

MacinTek

Thanks for your responses. Umm.. I tried disconnecting the idle stabilizer prior to starting the car... however, the engine was already warm. Whe I said the idle fluctuated... I meant.. it cycles from 1000rpm to 1800rpm and back every three or four seconds. I also feel the affect of this malady whendriving at low revs.
I did test the TPS... the proper poles for closed throttle showed infinte ohms.However, when I tested the other pole (throttle open), it didn't register at all. Can I assume my TPS is toast? Would this cause the cycling throttle surges? Any suggestions are welcome.

Thank you to all who offered suggestions.

nwehtje944

I would go for a bad idle stabilizer. but there are some more knowledgeable people on this board than I, so I would wait and see what they had to say.

Danno

A rhythmic idle like this is indicative of an intke leak after the throttle-body. And also a sign of a good idle stabilizer. What happens is you have a feedback loop that's out of phase. The intake leak causes the air-mixture to be too lean and the DME adds fuel. Which raises the idle further. Then the DME tries to slow it down with the stabilizer which cuts off the air, but causes a too rich mixture. Which kills your idle speed and it drops suddenly. Then the DME tries to stabilize the drop by opening the stablizer, which picks up the idle again, causing a lean mixture.

If you have an air-fuel ratio meter, it's interesting to watch this game, kinda like a concert. The tach does it's conductor up & down rhythm while the air-fuel ratio meter follows.

So, track down all the hoses and stuff under your intake manifold. Most likely one of them has developed a crack and is leaking. Or the intake manifold gaskets themselves could have developed a leak.

DLarsen

I've experienced this problem twice in my 17 years owning my 86 944 na. The first time (10 years ago) I replaced the idle stabilizer and the NTCII sensor (mounts in the cylinder head). This corrected the problem. The second time occurred several weeks ago (during the cold weather here in the northeast). In addition to the idle surging; when I would come to a stop on the road at a light, I normally would push the clutch in. My RPMS held at 2,000 or sometimes at 3000 steady. Anyway, at 243,000 miles I suspected a bad vacuum line (under the intake manifold area). I use my 944 daily for work and wanted the Dealer to check it out. They couldn't duplicate the problem (as it was intermittent for me). I asked them to remove the intake and replace several vacuum hoses (to/from the idle stab valve, oil breather hose). I also had them replace the throttle sensor switch(original) and the NTCII sensor. My idle stab valve is about 1 year old, so I didn't think that would go bad so soon. Well, had the car for a week now and it runs normal. If this condition should return, I might look at replacing the AFM (still original).

MacinTek

LOL!!! I enjoy a good concert as much as the next guy but... I prefer it in a concert hall or Stern Grove, not under the hood of my car.
I guess I'll have to break down and take the intake manifold off and replace all the hoses under there. Al? Rob? Jeff? if yer readin' this, I'll be over to Parts Heaven in about 90 minutes!
Thanks again for all the suggestions.

MacinTek

Hmmm... not good.
Last night I completely removed the intake, inspected the hoses and vacuum lines, replaced the TPS with one that tested good, and replaced the hose between the idle stabilizer (which was cracked) and the manifold and this morning... no difference.
I'm beginning to think it's the AFM... but then I could be chasing my own ignorance again. HEEELLLLPP!!!

MacinTek

Okay... next post.. I toof the intake assembly off... again... Which doesn't seem like a mjor deal but... that Racing Dynamics strut tower brace has to come off before I can do anything around the manifold. Anyway, per recommendation of some listers who were kind enough to advise me... I replaced the idle stabilizer with a spare (used) one I had lyin' around from another engine.. don't ask.
It was still a bear to start up... about two to three minutes of cranking attempts... but once it did start... the damn thing ran fine! SO... that issue seems to be resolved. Now... why the cold starts? any ideas?