84 Carrera 3.2 strange: engine/DME issue

[ischmitz]

I have a real strange issue with a '84 Carrera 3.2 car. Here is what happens:

1st scenario:
When I take the engine in second gear close to the rev limit the engine shuts down completely. Going downhill I can coast to almost a stop with the clutch engaged all the time so the engine slowly comes down in RPM. At no point do I get anything when stepping on the accelerator. And once I get close to a stop and disengage the clutch the engine is off.

When I turn the ignition key to OFF I can re-start the engine and it runs fine just fine.

2nd scenario
Again I take the engine close to the rev limit in 2nd or 3rd and it shuts down. (no power when I step on it) I then turn the key to OFF while the engine RPM still slowly comes down with the car coasting and the clutch engaged. (Don't try this at home because the steering wheel lock could engage )If I can keep the key in the OFF position for at least 5 seconds and then turn it back to ON the engine (still being dragged by the coasting car) comes back online. If the time in the OFF position is less than a couple of seconds and I turn the key back to ON the engine makes one attempt to fire and then dies.

So in summary once the engine dies by being taken close to the rev limit I need to power-cylce the DME with at least 5 seconds off-time to get it back online. The exact RPM where this happens can vary from 4.5k all the way to redline.

I have thrown in a known good 3.2 DME: same behavior
I have thrown in another coil: same behavior
The ignition cables look very new (no cracks) and so does the cap
I have not changed the DME relay (maybe something to look at)

I then measured the resistance of both the speed and the reference sensor. While one comes back as expected (1.2 kOhm between the sensor) and nothing to shield the other gives some really funky numbers. I do not get anywhere close to 1.2kOhm it's more like 300kOhm. It was dark but I tried many times to measure and am reasonably confident my measurements are correct.

Now here is my question: Could a sensor (it's the plug in the middle below the white CHT) be bad and the car still runs fine but craps out like I describe?

What makes this so strange is the time required to have the key in the OFF position for the engine to come back online and that it happens 10 out of 10 times.

I am stumped - any thoughts out there?

Ingo

[pozee]

Wild guess here - have you given any thought to the rotor?

[ZOA NOM]

DME RELAY, absolutely.

[Zeke]

How does the DME relay know what RPM or speed the car is doing?

[2.70Racer]

ischmitz,
Last spring I realized my '85 Carrera was 23 years old. I bought it in 1999 and I don't have any reciepts with it.
I decided to get proactive and do a little preventive maintenance.
I replaced the fuel pump, DME relay, and the crank shaft sensors.
Plugs, rotor, wires and mass air sensor are fresh.
It's time to get ahead of potential problems.
I don't like sitting on the side of the road.

[CliffBrown]

Fuel starvation?

Fuel filter?

[dimeified]

im thinking fuel as well. When you turn the key off for 5 seconds to let the rpms drop i dont think your resetting the dme as much as you are bringing the motor down to lower fuel pressure levels which the pump/filter can handle? But that doesn't necessarily mean there is anything wrong with the fuel system. You already know the sensor is not within an acceptable range of resistance, and that would give out of spec readings to the dme, which could possibly cause the dme to provide the wrong amount of fuel.

[ZOA NOM]

Originally Posted by milt

How does the DME relay know what RPM or speed the car is doing?

It doesn't. There are likely hairline cracks in the solder joints on the DME relay that will flex under the vibration caused by acceleration. Besides, why would any Carrera owner NOT change the DME relay first?

Change the relay, then look further.

[ischmitz]

I first thought fuel starvation as well but there are two observations that do not mash with this theory:

- I can run under load pretty hard all the way up to 4k any day and nothing happens
- Coasting without turning the key to OFF and lifting the throttle all the way never gets the engine back. I'll have to come to a complete stop.
- Coasting and turning the key to OFF for 5 to 10 seconds does bring it back every time. I used the same stretch of road to repeat the test several times.

In any case maybe it is time to invest into a fuel pressure gauge with a long extension so I can read pressure levels while driving.

The owner of the car had a major tune-up some 5000 miles ago where lots of comon things (cap, rotor, plugs, plug wires, fuel filter, etc) all were replaced for a hefty penny. He is not even sure when the car started having this issue since he never took the RPM's that high initially. I know that deserves a thread (and beating) on its own, but it is what it is. So for all intents and purposes this could have been going on for a long time.

I could see the DME relay since turning the key to OFF does cycle the first stage of the DME relay. So maybe that is something to swap anyway just to eliminate it. It could be a certain vibration (resonance of the body) that sets it off. Maybe I'll yank it and resolder it and clean the contacts just for the heck.

In general I am not a friend of randomly throwing new parts at a problem. That is why I didn't yet start to buy sensors and stuff like that. But the resistance readings for one of the sensors it really alarming to me. Does anyone know of the top of his/her head which is which. It goes like this:

Color - sensor - position on rail
white plug - CHT - top
blue(?) plug - sensor 1 - middle
black plug - sensor 2 - bottom

How can I tell which sensor is which w/o having to trace the sensor wires or measuring the harness. I looked in the WSM but could not find the designation.

I don't have the car with me know but here is a test that maybe someone could perform if he/she is bored: Run the engine and rev it up good, next pull the middle plug (sensor 1) and quickly re-connect it. I wonder if that would kill the engine or if the engine would catch immediately, once you re-connect the sensor while the RPM's haven't gone down too much yet.

What could be wrong with the rotor pozee?

Ingo

[burgermeister]

Wouldn't the DME Relay get used to shut down the engine as the rev limit is reached?

Does this happen in neutral (while standing still) too, or just under load? i.e., is it RPM or Load related?

[burgermeister]

Wouldn't the DME Relay get used to shut down the engine as the rev limit is reached?

Does this happen in neutral (while standing still) too, or just under load? i.e., is it RPM or Load related?

[E Sully]

I would think it's a sensor. You got bad readings on one of them. If it was the speed sensor I'd change that first. Speed sensor is labeled DG.

[ischmitz]

Originally Posted by burgermeister

Wouldn't the DME Relay get used to shut down the engine as the rev limit is reached?

Does this happen in neutral (while standing still) too, or just under load? i.e., is it RPM or Load related?

Not really - the 3.2 DME just cuts spark when you hit the rev limit. The DME relay stage 1 is on as long as the ignition is on. Stage 2 is on either during cranking or during engine speed > 400 RPM.

I did try reving the engine in neutral but could not reproduce the effect. So it seems I need to be under some load. It does happen in 1st, 2nd, and 3rd. Didn't try higher gears/HW driving.

Ingo

[ischmitz]

Originally Posted by E Sully

I would think it's a sensor. You got bad readings on one of them. If it was the speed sensor I'd change that first. Speed sensor is labeled DG.

where is the label found? On the connector?

[E Sully]

There should be a collar tag on the wire behind the connector.

[ZOA NOM]

Seriously, do nothing before you swap the relay. These are legendary for exactly your symptoms. Buy an extra one to keep in the glove box.

[Joe Bob]

My first guess was the rotor as well, assuming it was a rev limiting model with a weight in it, like the SC, since this was an 84....as of last night I think they narrowed it down to the sensor?

[dfink]

If you don't want to buy expensive sensors you could try to just swap them and see if it helps. If so then replace them. If they have never been replaced though they probably won't take kindly to being moved and probably should be replaced anyhow.

[mjshira]

my first rgruppe car was a 69 with a 3.2

it had some strange issues like what you describe (not exactly, but similar...)

I chased this issue for many months and many parts. Then finally I swapped DME's and it stopped. Turned out that the DME I had had gotten wet at some point in its past and this impacted how it could deal with heat... so when the DME would get 'warm' or if it couldn't dissipate heat, the issue would happen.

So I would try and second DME and see what that does

[Steve W]

I don't remember if we discussed this before, but I had a customer that burned through three rebuilt DMEs under rally racing conditions. Couldn't go more than 5 minutes into a race. Cost was no object and he went through the entire motor and replaced every sensor and component in the entire motronic system. I sent him a virgin DME from an 89 Carrera thinking it might have been the quality of his rebuilt DMEs. Two hours of hard driving through his countryside roads and no issues. So thinking problem solved, he went 5 minutes into his race and the DME burned up and shut down. The issue ended up being bad fuel injector(s) that surfaced itself upon overloading the DME at high duty cycles of the full load conditions during racing.