StanUK951

Hi all,

I'm posting this (long) thread in the hope that somebody might recognise the symptoms of my misfire and point me in the right direction... I've had the car about 8 weeks and have had the problem since about 2 weeks after buying the car. It's a 1991 Carrera 4 with 102K miles but a freshly rebuilt engine.

Breakdown 1: I was in stop-start traffic and noticed the car start to misfire at idle. I tried to blip the throttle to clear it but it seemed to flood and died. It restarted fine but was still misfiring. I could rev up to about 3-4000rpm and it would misfire all the way but would cut out after running for a few seconds. I coaxed the car to a side road and eventually it ran fine and the misfire completely cleared up. I drove straight to the independent Porsche specialist I use and they suspected the DME relay (which was the old type apparently) and replaced it for me.

Breakdown 2: The misfire came back about 2 weeks later exactly the same as before. Tried restarting a few times but it would misfire and backfire through the exhaust badly. I swapped the DME relay round for the old one I still had after having left the car for 10mins or so and it started fine. I then swapped back to the new relay and still everything was ok again. I put it down to bad luck with the DME relay and made a mental note to buy another one.

Breakdown 3: A couple of weeks later again and the misfire was back (I still hadnt got round to replacing the DME relay). I swapped the relays around again hoping that would cure it but a mile up the road it started misfiring and died again. This time I couldnt coax it back to life before the battery went flat and I called the RAC. They put a power pack on the battery and it started straight away and was fine again. The breakdown guy tested the voltage across the battery and thought it looked a bit low. We left the power pack on and drove back to the dealer I bought the car from (as I have a 6 month warranty) and left it with them. After explaining all the issues they suspected the fuel pump and duly replaced it under warranty.

Breakdown 4: On the motorway (highway for our US cousins) 1 day after the fuel pump was changed the misfire was back with a vengance and the car cut out completely whilst I was in the outside lane. I managed to get over to the hardshoulder and tried to restart the car. This time the misfire was different. It would run but it was very rough. The engine didnt die but it was undriveable. After waiting half an hour hoping the problem would clear again I tried to restart it but it would still misfire and run rough so I admitted defeat and called the RAC again and had the car towed straight back to the dealer that night (I didnt try swapping the DME relays again this time though). When they started the car the following day it was fine again. They replaced the fuel pump again and swapped the air flow meter from another car. They checked all the 'usual parameters' on the hammer and everything seemed ok. They also checked the recharging voltage and said it was 0.2V down on spec but didnt think this was an issue (I know the battery is fairly new). The mechanic also gave me a spare DME which he knew to be good when I picked the car up last Friday.

Breakdown 5: After driving a good few miles over the weekend but not at all during the week I drove the car this morning and it started misfiring in almost exactly the same place it did in breakdown 3 (just half a mile or so from my house). Again I managed to get to a side road and tried restarting a few times with no change. I had to take my daughter to nursery so I walked her the rest of the way and when I came back to the car after 10 or 15 mins I tried swapping the DME relay again with the good one I had from the mechanic. It started straight away and ran fine on the way back home. When I got home I swapped the relays around again and it was fine still with the other relay. This was this morning, a week after picking the car up from the dealer for the second time).

If you have stuck with this saga so far thank you very much! If anyone has any (useful) suggestions I would glad to hear them. Am I barking up the wrong tree with the DME relay? If it happens again with the mechanics relay then I will know for sure but it hasnt yet... Could it be the alternator under charging causing funny electrical gremlins - I know 964's are a bit sensitive to this sort of thing...? Other suggestions from mechanics at my specialist and the dealer have been the DME ECU itself or maybe the aftermarket alarm wiring (if its connected to the DME relay) but unless the fault happens with them they cant easily find the problem.

All suggestions are greatly received! Thanks.

DaveK

From what you've said, I don't think it's the DME relay at all. Same problems with both relays rules that out. In fact - if you want to be sure and you're anywhere near Camberley (M3 J4) you can swap it with the one in my car.

There is a TSB for the wiring to the air flow meter. I have had intermittent hesitation problems on and off for years. Not nearly as bad as you have - but it hasn't happened since I checked mine earlier this year and found it hadn't been done. I did it myself and so far it's run fine. See post 4 here :

https://rennlist.com/forums/964-foru...rness-mod.html

Other than that - I'm not sure. I had idle and stalling problems when I bought my car - was worn distributor caps / rotor arms. A lot of dealers seem to forget to check the basic stuff (took a while to get it fixed). I'd definitely check them - they could cause your problems.

Coils could also be an issue I would think - might be worth trying to run on one coil at a time (disconnect at the coil end, not the distributor end).

I would definitely try stuff like this first before swapping fuel pumps, AFMs and DMEs.

Edit : In fact - I do remember having symptoms a bit like yours where the car did die a couple of times (not at idle) years ago. I can't actually remember how I cured it though!

Wachuko

Are you sure you are running on both distributors?

I second DaveK's suggestions... Start there.

crg53

I am no expert on these electric gremlings, but definitely try to run the engine on one and then the other coil. Mine had a problem with backfiring and stalling, and it was one of the coils, disconnect them as the ignitor.

StanUK951

I am doubtful of the DME relay being the fault too, it just seems a strange coincidence that on two occassions messing around with it has 'cured' the problem... They did check the distributors, spark, etc. when it was in the last time. Sounds like it would be worth looking at the coils though and see if unplugging one recreates the symptoms that I have... I will do that tomorrow. On a car with good coils, what would I expect to experience disconnecting one? Would there be a misfire or would it just run a bit rough...?

I will try that TSB fix as well, thanks Dave.

Thanks for the quick responses guys. I am going to become a member to support this forum - the advice and help is great!

crg53

The engine should run/sound the same on either coil, i.e. run smooth, if it does not, you have a problem on that side of the ignition system; it could be the coil, distributor cap/rotor, or a spark plug, or a spark plug wire, or any combination thereof, but in your case the symptoms point towards the coil. Let us know how you make out.

Wachuko

And be careful when working those cables... they bite...

tbennett017

Someone else had running problems, and traced it to the stock chip being seated improperly. Check the fat cable to the box under the driver's seat, and if you are intrepid, open the computer box and ensure that the chip is seated firmly in its socket...

springer3

Intermittent problems are the most frustrating. +1 on all the suggestions so far. Not mentioned yet is fuel contamination. Replace the fuel filter, and cut open the old filter. If it is full of water or dirt, you will need to drain the fuel and clean up the suction strainer. A previous carbureted 911 I owned had periodic episodes, and it was dirt clogging the suction strainer. When stopped to fidget with something, the sloshing fuel would clean off the screen, and the fuel pump would get fuel until the strainer clogged again. The shop should have caught this when they replaced the fuel pump, but possibly it was missed.

Inspect the caps and rotors. Arcing inside the distributor cap causes mis-fires and could even fire the wrong plug make for very poor running. How do the spark-plug wires and coil wires look? You can see arcing if you open the engine bay on a dark night and turn off the engine bay light. Fog the area with a pump sprayer and you will see a light show if the wires are breaking down.

Have you read the diagnostic codes?

darth

Check the connections the relay plugs into, make sure they're all tight and if not try getting in with a pair of needle nose pliers and squeeze them together to make a tighter fit/connection.

Hope this helps
Bill

StanUK951

Thanks for all the ideas guys, I will try all of these and might just replace the coils, dist caps and rotor arms anyway for what they cost.

As my fault is intermittent, would the problem show up when I disconnect the coil or would the fact I am disconnecting one coil put more load/strain on the other side and show up any weakness??!

I've had the ECU apart already and replaced the chip with a Roock chip as well so at least I can rule that one out but thanks for the suggestion!

How can I read the diagnostic codes Springer??

Indycam

You have two sparks per cylinder .
The motor needs the two sparks to run well .
The motor will run on one spark and that can mask the fact that one spark has gone **** up .

Do you know how to check for spark ?

darth

I can't imagine running on one coil would produce optimum performance as the secondary distributor's timing is offset, I believe intended to produce complete combustion, the same principle employed on Mazda' s rotary Wankel engine. I know when I disconnect the secondary coil on my engine it doesn't run quite as smoothly as running on the primary coil and runs worse when only the secondary is connected, it is only when both are connected that I get a smooth running engine, yet after reading other Rennlisters experinces they don't mention the same.

Bill

Indycam

"as the secondary distributor's timing is offset"
The sparks go at the same exact time .

StanUK951

Hi Indycam, no I don't know how to check for spark?