Consistent intermittant brief engine shutoff
 

1991 C4

My car has a consistent intermittent and brief shutoff problem. It will shut done for less than a second, the check engine light flashes on and as long as the engine has momentum it will continue. The car jerks. The person following me to the shop noted black puffs of smoke each time the on/off occurred.

The shop I took it to is good and the techs are smart and experienced but I would like some additional opinions if only to learn.

The have diagnosed a failed DME and suggest a $1,600 Bosch re-manufactured part.

During the diagnosis they placed an oscilloscope on the leads from the different inputs (cylinder head temp, air flow, etc) and the signals were normal. The placed the scope on the outputs and the signal was normal then would just shut down for the split second when the engine died. Bottom line input good output bad. I do not know the details of this test just that it was done with an oscilloscope attached to an inline pigtail to pull off the signal.

They are 90% confident that it is the DME but the problem is that if it is replaced and still does not work I have just wasted that money and of course the cost.

While I was there they pulled the DME and examined it and also replaced the performance chip (which was put in by the previous owner) with the original one that i got when i bought the car. There was not change.

Any help for the community would be very much appreciated.

Thanks,

Matt

i try to use a loaner unit before shelling out big dough.

I agree with John. You should ask them for loaner before paying $1600 for a replacement. Give us some more details:

How often does this happen?
Did it simply start one day?
Has the frequency of ocurence changed?
Does it get worse with time?
Is it worse when it's wet outside, cold, dry, engine cold or warm?

Has the shop checked that the DME relay is sound and is not causing the restart? From your description the DME essentially reboots. This can be bad power (permanent +12V or DME relay) or an internal issue in the DME.

964's are known to be very sensitive to all sorts of electric interference. Another culprit believe it or not could be a bad spark wire. When spark plug wires become brittle they can arc and cause enough interference to cause poor running. You should take a look after dark in the engine bay with the engine running and check if there are any signs of the wires arcing. Just a thought.

I have a spare 964 DME that I would be willing to sell in case you have to go down that road. What is the model number on your DME. It should be something like 964.618.124.xx Let me know. Maybe we can come to a deal where I get your broken DME plus cash.

Cheers,
Ingo

I had the same issue a few months back. It turned out to be an ignition module and coil. My 95 only uses one module but I think the earlier ones used two. The problem stemmed from a bad ignition coil which ultimately caused the module to overheat and finally burn up.

Even after replacing the module it was too hot to touch and that is what got me looking elsewhere to ultimately find one coil bad. It could have been the coil or the module causing the "hickup" but it was like someone turning the ignition switch off and back on again. Once replaced, no more problems. (ran better too)

Make sure both sets of plugs are firing. Easy test.....

Thank you so much for the replys.

Some more information as Ingo requested.

Over the last year or so I had a problem with it running poor, twice I believe. The shop connected it to their diagnostic computer and it returned a problem with the air flow indicator. There was no repair done and we thought it was a bad connection or a bad poteniometer that just started working again.

About a month or so ago I was on the freeway at speed and it just died. I coasted into a parking lot and tried to start it. It would sort of idle but would not run. I reached around the back and wiggled the wires connecting to the air flow sensor and really just tried to get the car to run by operating the throttle from the engine compartment.

Somehow that fixed it and it seemed to run fine for a week or so then the problem started again. I parked it over christmas and last week (after changind the battery) was hoping it would be OK..... dreaming I know.

This time the failure was "consistently intermittent" as previously described. When the shop attached the diagnostic computer it gave roving (as they called it) indications of failure. I think it was the air flow sensor again as well as the throttle position swithches though i cannot confirm this. I was watching as this test was being conducted but was not doing it myself. It seemed to be giving lots of ambigous results.

Again, the techs placed an osciliscope on the inputs and outputs and told me the inputs were good and the output dropped off when the car hesitated.

I agree with John that the best thing to do would be to try a DME that is known to be good but niehter I nor they have one.

Ingo, would be willing to work out an arrangement where I could "rent" your DME and if it worked then buy it. Also, since you want to trade (along with cash) for mine I assume you would then fix the one I send you.

Regarding the response about the ingition module and coil. I am not sure if that was checked by the shop but believe it was.

Thanks for everyones help. It really means a lot.

Matt

Thanks for all the help.

It turned out to be as was suggested a bad coil.

Anyone want to buy a good used DME :(

Thanks,

Matt