ToreB

Hi guys.
I really struggle with this one and need help. 1994 993 with Porsche Drive Block immobilizer.
The car stalls unexpectedly, at cold or hot engine, on long or short journeys. When this happens, the starter cranks, but no engine start.
The owner then waits, or fiddle with immobilizer/ignition, and then it suddenly fires. I've driven it for several days now, and haven't been able to get the fault to appear. No fault codes except for an occasional O2 oxygen sensor fault. This is the early immobilizer with no warning light in the dashboard clock. I suspect an electrical fault, the owner thinks she smells fuel when trying to crank the engine.
Ignition switch should be OK, since all other Terminal 15-powered units are working fine when this happens. All fuses (including main 60A fuse) are tested OK and no intermittent contact. (sometimes seen i the 964)
So far:
-New DME relay (new relay verified OK)
-New crankshaft (flywheel) sensor
-Bypassed Terminal 15 relay (DME power supply) in Alarm control unit
-Aftermarket DME chip changed to original EPROM
-I have wiggled and pulled all wiring/relays I can reach in the engine compartment, fuse boxes and under both seats. (alarm/immobilizer/DME) No signs of moisture or corrosion. Grounds OK.

There may be a fault in the through-pass inside the air bag control unit for the DME Terminal 15 power supply. However, I haven't seen notes that this has happened before.

I am stumped, and hate throwing in the towel. Maybe I am wrong in assuming this is a electrical fault. Any suggestions?

Cheers,
Tore

pp000830

Running needs fuel and spark, does your customer hear the fuel pump running?
Can he check one of the plug wires for spark when the issue crops up?
This may give you some direction.
Also is the battery at at least 12.3 volts or is it running lower as in the DME can get flakey if the inadequate voltage to it.

As much as I hear about immobilizer issue on this board, I have never had any to speak of.

ToreB

Thanks pp000830.
Battery is OK, engine cranks fine.
I have tried to provoke the fault for several days now, and have been driving around with voltmeter, spark plug tester and PST-2 diag system ready in the passenger seat all the time. But no luck, running like a kitten.

I'll get her to listen for the fuel pump humming.
Cheers,
Tore

gavonder

Any chance the battery had been recently disconnected?
I had a similar issue after swapping my steering wheel (disconnected the battery for a couple hours). After driving it a few times, I guess the computer "relearned" and it has been fine ever since.

Sandia man

IF it has the old flat battery ground strap, change it. Cheap and very easy to do. It seems the old type developed periodic conduction problems.

nile13

Tore, there are several things here that are quetionsble. Let's try to separate:

1. Stalling. A car will stall if the O2 sensor is lazy, i.e. doesn't respond fast enough. it'll mostly stall when coming from lean condition, like cruising on highway with part throttle and getting off The ECU doesn't react quickly to provide richer fuel/air mixture, bingo, car stalls. So I'd seriously consider replacing O2 sensors if the car is well above 100K Km.

2. No start. Does the engine crank and continues to crank, or does it crank and than makes a metallic noise, almost sounfding like there's a stripped gear? If so, I recently fixed this very issue with a new starter. Apparently, the starter shaft gets gummed up and starter doesn't engage. A friend had a similar no-start issue but I don't believe his car would crank at all, the suddenly would. Also a new starter. Both of us experience that when car is hot, it would start fie after a bit or cooling. While you can't replicate stalling, you can check this condition easily by shutting the car down after a bit of driving and see how it re-starts.

3. Immobilizer in a '94? That's odd. Only very late '95 got immobilizers in US and I thought that was worldwide?

PS. I feel very odd as it's quiet preposterous for me to give electrical-related suggestions to ToreB. Hope it's just another set of eyes looking at it, Tore, that might bring a fresh idea to the situation.

Steve Weiner-Rennsport Systems

The owner's nose is a clue that should not be overlooked.

Assuming the Immobilizer has not disabled the fuel pump and DME power, I would look for an ignition fault. Unless the problem occurs on your watch, it going to be tough to check for spark when the problem is occurring.

I've had issues with the head temp sensor and crank sensor connecting spades backing out (inside the connector) and this is not apparent to the eye.

Naturally each & every ground in the engine compartment is suspect and all MUST be bright, shiny & clean.

Vorsicht

Does the ignition switch provide power to the DME relay? If so, could that being bad cause a stall?

nine9six

I think Peter is on to something here...^^^
Intermittent ignition switch issue... could it be the ignition switch is on its way out?

bobboinski

When the owner left the car with you, you probably only got the ignition key. Wonder if in daily use there is a big heavy ring of keys dangling from the ignition switch. Might make a difference in diagnosis?

Steve Weiner-Rennsport Systems

The DME gets switched power, yes. Stalling would only occur if the power to the DME became intermittent for some reason.

user 723923490

You indicated the DME was verified OK. Is it a Porsche DME?
URO parts DME is garbage.

ToreB

Thanks all. It is always helpful to get other opinions in such difficult cases.

The '94 993 is a bit odd from the rest, also the immobilizer. This car was sold in Germany.
Battery connections and wiring are OK, also the ground strap. Starter runs normal. DME relay is Porsche replacement part, and is fine. As mentioned, the ignition switch is probably OK, since all other devices are working while the fault is present. I am using the same key as the owner.
I have little experience in the engine sensors, but expect a failing head temp and and crank sensors or their connectors would produce a fault code in the Motronic unit.
I also have little experience in the lambda/O2 sensors, but have read that the engine management will run on a fallback mode if they fail. I therefore do not think this is the issue. I would expect the engine to be possible to start immediately after such a stall, it is not. However, I'll keep an eye on any reappearing fault codes.
I agree with Steve, this is most likely an ignition issue, maybe a failing Ignition Control Unit. I gave it a bashing and exersized its connector while the seat was out, but you never know. I hate throwing in new parts in a car without being able to properly diagnose the fault.

Cheers,
Tore

Endoman

Similar problem on my '94 was the crank sensor. That would fail when hot, restart after a couple of hours then fail after 15mins running. 4th one I have heard of recently. I don't know if it gave a fault code but OPC spotted it very quickly.
The 944 had a faulty check valve in the fuel pump, poor starting especially when hot, occasional misfire and stall, easily checked by fuel pressure leak down test

nile13

Tore, if it's an O2 sensor issue, the start would be immediate That's why i mentioned that there would be two separate issues if stalling was indeed due to O2 sensor reacting slowly. The sensor is fairly easy to test with an oscilloscope. But also driving with very partial throttle and coming to a stop is easy to check - it might cause a consistent stall.

O2/Lambda sensors are a wear items and need to be replaced every 60-100K miles anyway. They are inexpensive and I'd replace them.