Synapse120

Car info: 1995 993 6MT 101k miles.

Background into the problem:
I had the car sitting for 6 months. when i last drove it, I parked it with 1/4 tank of gas. Prior to sitting the car ran perfectly. I didn't have the car on a battery tender so i trickle charged the battery over night, and after about 3 attempts the car started and seemed to be idling fine.

After about 10 minutes of idling i took the car for a drive (or tried to). I got to the first stop sign and the car stalled when the coming to a stop (revs dropped). The car wouldn't start, so i clutch started it since i happened to be on a hill.

The car ran rough and had some hesitation as i drove down the highway, and any time i had to come to a stop the revs would drop, and if i didn't keep the revs up it would stall.

As i got back into my neighborhood someone walking yelled that my car was on fire. I got out and sure enough the exhaust pipes were on fire, as well as the bumper at this point. I put the fire out, and saw some smoke from the rear passenger tire well (that eventually went away). Raw gas in the exhaust is the only reason the exhaust pipes would be on fire, now to determine why this is happening.

I then had the car towed to the dealer

Present day info from the dealer:
The dealer checked out everything or so they tell me, for two weeks they said nothing seems wrong with the car, and everything checks out, except the battery, so that was replaced, and they continued to check over the car.

They come back to me and tell me that the ECU is bad causing the fuel injector in cylinder 6 to remain open at all times pouring gas into the cylinder, and eventually into the exhaust. They tell me the root cause is that i need to replace the alarm control unit, part # 993.618.123.02 which to my luck is a discontinued part, and all superseded parts are discontinued except 993.618.123.GX which costs around $5k.

This is where i'm questioning whether this is the actual problem or a result of the fire and extreme heat that was in the rear of the car. I'm also curious how a control unit would go bad after the car sat for just 6 months.

I'm looking for advice and ammunition to go back to the dealer with to confirm that this is the only problem with the car. If someone has some knowledge of the fuel injection system in these cars and could help me out that would be great.

KNS

My first guess before you mentioned the fuel injector was that you'd had an oil leak, perhaps a valve cover or power steering leak. Sitting so long caused some oil to pool up on a heat exchanger or something and it finally ignited after getting hot enough. Not uncommon at all, it's happened before.

Older 911s (and many other cars) had CIS injection in which the injectors were squirting continuously (if I'm remembering their operation correctly), they didn't catch fire.

chaoscreature

When I started reading this thread I thought the same thing as KNS. Are you sure that the flames were coming OUT of the tailpipes and not OFF of them? I have had some pretty smoky moments from oil leaking onto the pipes, and once from me leaving the oil cap off

When you started the car, did it take a jumper of any sort? Your battery voltage could have been too low at idle to keep the engine running?

I have seen flames coming out of exhaust before due to excessive fuel, but it usually makes some pretty interesting sounds either like a jet engine if there is enough of it or more of a crackling if less, or with a BANG if due to a backfire.

I hope you get this resolved, please keep us posted!

techman1

Fuel in exhaust, possible. Blocked exhaust with rodent nest, possible. Oil on exhaust, possible.

Did the DME go bad, or did injector 6 stick open and? Did a short in the wiring, caused by rodent chewing, take out the DME?

Taken to the dealer, expensive, but hopefully correct diagnosis. We can make guesses, but need hands on to be correct. If you trust the mechanic, they are usually the one with the opportunity to put hands on and do an analysis of the evidence.

Regarding the DME, I believe a Rennlister has come up with a plug in replacement for the DME, which I believe the part number you quoted identifies. That part number is for DME, not alarm unit.

Synapse120

The actual tailpipes were on fire, it wasn't flames coming from inside the exhaust, they remained on fire after the car was shut off. The car was running rough, and hesitated quite a bit.

I saw no evidence of rodent nests, in the car or in my garage before starting the car.

The battery was reported to be bad, and to note the radio wouldn't even turn on when i first started the car, so low voltage coming from the battery is very possible.

The dealer is telling me the DME is bad, and that is why injector 6 is staying open due to a bad signal from the DME. I've asked them how they determined it was a bad DME and not just a stuck injector, for which i'm waiting on their response.

NP993

Doesn't make sense as I don't believe the DME controls each injector individually.

As someone asked above: was the fire coming from inside the tail pipe or from outside it?

jscott82

Id want to see an oscilloscope trace of the injector.. That is the only way you would convince me.

If you want a second opinion, I would have Jim Burton at Hendrick Porsche take a look. I haven't met the new service writers there so I cant speak to their character, but I know Jim is a stand up guy.

If you want an independent: Lou Smith at Exclusive Motorwerks is another good resource around here.

As a last resort... I'm in South Charlotte and have a scope we can use...

IXLR8

There are separate lines going to each injector from the Motronic.

Synapse120

The fire was outside of the pipe (and the bumper), and some on the inside of the pipes. In both cases it was on the exhaust tips only. Also both rear fenders were extremely hot to the touch.

Jim is the person that has been looking at the car at Hendrick porsche, however i haven't spoken to him directly.
Where in south Charlotte is your shop? I'd need to have the car towed there since i don't want to risk any more damage.

Steve Weiner-Rennsport Systems

Your 993 has sequential fuel injection which means that attaching a "noid" light to each injector connector should result in a flash, showing the injector pulse.

If the noid light stays on, that injector driver (inside the DME) is stuck. If the noid light simply flashes, the injector itself is the culprit.

I'd ask them about all this.

jscott82

Sounds like you got the right guy... I would find a way to talk to him directly.

If it is the DME, I would call Downtown Radio Service (here in charlotte) they do board level repairs. Not sure if they can do an ECU. If it is truly a failed driver it should be fairly easy to trouble shoot and repair though... For 5k its at least worth the call.

No shop.. Just my garage, which is full of junk at the moment as we remodel the kitchen...

techman1

Here is the thread regarding a replacement DME
https://rennlist.com/forums/993-foru...er-1000-a.html
Interesting read.

IXLR8

You can easily confirm that with a LED and series resistor.

Davies

I've often wondered if the oil that pools on the heat exchangers from leaky lower (and upper) covers could get hot enough to catch fire. It certainly smokes alarmingly if there's any oil back there. Leaked oil also does migrate over time to the rear of the exhaust/manifold/tips etc. when the car drives at speed. Which could explain the exterior of the tips burning. That does seem (to me) to be more plausible than a gasoline leak, which could be more likely to result in a very serious fire.

Steve Weiner-Rennsport Systems

Hi Matt,

To date,...I've never seen an oil fire originating from accumulated oil inside the heat exchangers. Thankfully I suppose, the smoke alerts the driver that something is quite wrong.

Oil fires from a broken oil line or connector; that has certainly happened so its prudent to keep a sharp eye on all that hardware.