Teach me about DME

[Patronus]

Being a CIS guru, I find myself lost in the new world of DME.

My '85 Carrera (231hp Euro) is sputtering along, and in a previous post I asked opinions. It acellerates beautifully and the same goes for decelleration. But if I keep my speed constant, it seems to cut out about 3 times per second, randomly. Like a serious misfire. It idles smoothly. The car has new injectors.

I got some good advice from the previous post, but I don't know how to go about following it.

First dumb question, where is the O2 sensor? I need to see what happens if I disconnect it. That's if I have one. There is a heat shielded wire in the region of the exhaust, but it seems to be disconnected. I suspect it could have something to do with the O2 sensor.
Is the O2 sensor necessary in any event? I'm in South Africa and we have no emission rules (or at least, they never check it).

Secondly, I understand that there is a spring inside the DME system connected to the metering system. This spring tends to weaken, upsetting the mixture, and needs to be examined. Where is that spring, and how do I get to it?

Please, I don't see anything I can adjust, fiddle with, or even unscrew. Help!!!

[ChrisBennet]

Petronous,
The O2 sensor is located behind the left rear tire. A little shield should be attached to it to protect the rear tire. If your car was delivered with an O2 sensor, disconnecting it will make the engine run rich (and not in good way either). The O2 sensor is used to control mixture and has nothing to do with ignition or "cut outs". The O2 sensor is not used in full throttle operation.
I could write a Motronic FI trouble shooting checklist but I don't have time right now.
Because it is easy, I would start by testing whether the head temperature sensor is causing the problem. The head temp sensor is the white connector at the top of the 3 connector set on the left top of the engine, just to the left of the heater blower.
Run the car to operating temp and turn the motor off. Insert a 3-700 ohm resistor into the connector going to the car. Put a piece of tape over it to hold it. Drive the car and see if that fixes the problem.
-Chris

[Patronus]

Chris,

On the topic of O2 sensor - I don't see anything behind the rear left wheel, apart from a wire, enclosed in a protective white jacket. The wire is connected to nothing (seems to have been cut off), and is neatly tucked away to keep out of the way. You say that disconnecting the O2 sensor is a bad thing - should my car not perhaps have one, and it is disconnected/gone?

I will try testing the head temp sensor as described.

Clue: When the car is cold, the hesitation/misfire is much better, but it gets real bad when the car is hot.

[ChrisBennet]

Don't worry about the O2 sensor for now.
-Chris

[Don Plumley]

My '87 had a small stumble on hard accelleration. Ultimately it was a bad spark plug, for what it is worth.

It could be that your air volume meter has a "dead spot" at this constant speed position; but otherwise reads fine when the barndoor is moving the potentiometer. You can probe and check this easy enough.

The Bentley manual has a reasonable section on troubleshooting DME and nominal values for the various sensors.

[Patronus]

I ran the engine in the dark just now, and the plug wires seem to be arcing at a number of places. They must be really old and brittle (you can't really see it with the naked eye) but I can clearly see the blue indicators in the dark.

Can this cause the problem I'm experiencing with sputtering/misfiring at constant speed when the car is hot? I'm thinking, arcing plug wires should affect the car's behaviour when accellerating too, or not?

(Of course I'm gonna have them replaced, if you were wondering)

[Steve W]

You found your problem - definitely the cause of you problems. Think about it, if there arcing somewhere else, they're not arcing across your spark plugs. Idle and deceleration would not be a problem because the low combustion chamber pressures would not resist the ignition spark across the gap. However, under acceleration, you would normally notice significant missing and power loss, but the increase in ignition dwell is probably partly making up for that and masking the power loss, but then is where your wire arcing would be the most severe.

[ChrisBennet]

Arcing plug wires usually show up as a high RPM miss - which either you didn't mention or isn't happening in your case. I'm curious, does it run fine at 6000rpm?
-Chris

[Patronus]

I raced the car on Saturday and it was smooth, and kicked ass all the way to the rev limiter. But that's because I was always either accellerating or decellerating. On my way to the track and back from it, it was a different story.

The misfiring problem is not limited to low rpm, though. I'm sure if I let the car coast steadily at 6000rpm it would do the same, only less noticable because of the inertia of the flywheel.

My 2.7 CIS also had arcing plug-wires when I bought it, worse than this, and it misfired (much like the Carrera) up to 3000rpm, after which the engine smoothed out. It happened consistently, no matter whether I was accellerating or decellerating. But I'm wondering whether the DME system might be more sensitive (thus unforgiving) to electric fluctuations than the robust CIS.

Look, I'm sure the arcing plug wires is part of the problem. It might even be the whole problem, like Steve suggested. Unfortunately I won't be able to change them within the next few days, so we won't really know until then.

Speaking of plug wires - if I want to check the connectors that go into the plugs, do I just check their resistance? I assume they should measure 3000 ohm. Is this at any temperature? Also, what tolerance is acceptable in terms of their resistance?

Thanks for all the advice so far.

[Patronus]

I finally have the explanation, and we suspected it all along. New plugs and plug wires, and the problem seems to be fixed.