Antonio

Sometime last year my '86 951 track car started having intermittent drivability and idle problems. Basically the engine would start and run fine for a few minutes but as soon as it begins to warm up the idle will get really rough and the engine will begin cut out and backfire on throttle. Once the engine got above 3K RPM it will begin to clear out but still not run 100%. The symptoms immediately disappear almost entirely as soon as I disconnect the TPS. If I connect the TPS back sometimes the engine goes back to running rough and sometimes it continues to run fine. I have not been able to notice any pattern to this.

So far I have replaced the TPS, resealed the throttle body, swapped the DME, checked wires continuity, set the idle but the problem keeps coming back, not always but once in a while. Plugs, wires, rotor and coil are all fairly new. I've been running a Lindsey MAF and a Mufterburner fuels management system for a few years. I am not sure if they play a role in this.

I'm really perplexed of the fact that by disconnecting the TPS the rough idle and the engine cutting out go away. Any suggestions on what the cause might be or how to try to diagnose it will be greatly appreciated.

944Phil

Check the DME relay and check the ISV. Clark garage has the procedure to check them

Chris White

did you make sure that the TPS was adjusted correctly? You need to hear the click to make sure that its reporting back a closed condition. If not the DME will not go into idle mode.

Meine44

I have been having the same issues with my car.



Hopefully you can find the problem before me

Antonio

Phil, thank you. I thought the DME relay is binary. It either works or not. But its easy to eliminate that possibility. I'll bypass it and see if that makes a difference. I believe that I have a spare ISV somewhere. If all fails I'll swap it to see if that makes a difference. But I am not looking forward to removing the intake manifold at this point.

Chris, thank you. The TPS is adjusted correctly but I noticed that the throttle does not always fully return to close position. I need to look at this closer. Would an open TPS cause the engine to cut out and backfire to that extend?

I'm starting to suspect a wire connected to the TPS is sorting out somewhere. As I said, the problem goes away as soon as I disconnect the TPS.

944Phil

Actually, you should try unplugging your O2 sensor and see if it runs better. I was having a lot of issues with my car last week. The car wasn't idling properly, after some reading I unplugged the O2 sensor and it idles a lot better. The idle would only go wonky after warming up a bit.

I changed my O2 sensor tonight and now all,is good under the hood..... Almost.

Antonio

It is more than poor idle. The engine stumbles, cuts out and backfires like crazy. As soon as I unplug the TPS is all goes away.

dmcampbell

Reference sensors - replace. Mike

homerj

ill be curious to see what u find. mine has been doing this for awhile. will run great and then all of a sudden cough and choke. usually limp it home but sometimes it would just clear up. been solving other problems in the mean time so haven't been able to focus on it.

blown 944

Sounds like it might be the DME temp sensor. Easy one to swap out. That's where I'd start.

jerome951

With the car warmed up put a hose clamp where the ISV enters the intake manifold to test. Far easier than swapping it out.

Mine gave out and caused the symptoms you're seeing, but bypassing it electrically didn't work. It had failed and was letting air through even when closed.

Meine44

+1 on the DME temp sensor.


My car is apart awaiting AOS seals and all new vacuum lines (and rebuilt T-body), and I suspect the DME temp sensor will fix the issue.




But this assumes that the car idles okay. My car idles alright, but will jolt/buck/hesitate at cruising speed. Throttle it and it keeps running, but poorly at best.

mahoney944

Ive seen these symtoms happen from a bad ignition coil, or one thats going bad.

Meine44

Any updates?

Antonio

Thank you all for the suggestions.

I replaced the the DME temp sensor. I let the engine idle until fully warm. I statically rev'd the engine a few times. It ran good. No cutting out whatsoever. I am cautiously optimistic that the problem has been resolved. I will know for sure when the car hits the track in a couple of weeks and I'll report back.