!!--Motronics Experts--!! Unsolvable ELECTRICAL misfire NIGHTMARE!

[JiXer]

Ok. So... A little preface... I am VERY experienced with e30s. More than most people you know. But I have encountered a puzzle I cannot solve. I am at my wit's end!!!

The car starts, runs, and drives fine EXCEPT for a consistent electrical misfire which occurs on any given cylinder in a totally random pattern regardless of RPM or load.

The specifics... 1991 325i, manual, sedan. motronic 1.3 & 173 DME

Just so I am clear... this is a SPARK DROP! it is not a fuel thing, so unless you know of a way that an air or temp measuring sensor will tell the dme to pull a spark, there is no need to ponder on things like the AFM, coolant sensor, 02 sensor, etc. (which by the way have all tested fine anyway)

I have tested and swapped all the ignition related sensors, plugs, wires, cap, rotor, coil etc. and tested for continuity from every extremity to every pin on the Ecu plug. Grounds are good, crank wheel is fine, even swapped the alternator and so on.
The only thing I haven't done yet is swap the wiring harness itself as I have found nothing actually wrong with the wiring in any way.

I have also run an oscilloscope on the crank signal (both at the plug & inside the DME) and it is absolute and 100% consistent with no drops or blips of any kind.

I also ran the scope on the positive terminal of the coil and THAT is where I am able to see a drop in the signal that is 100% consistent with the stumble in the motor and the flash of the timing light. I then used the scope to probe the coil output pin inside the DME with the same results.

So the problem is definitely related to signaling the coil to fire. And yes, I have swapped out 3 other DMEs all of which run perfectly in their respective cars.

So the basic question is this... what possible causes could their be that would either tell the DME to stop sending current to the coil?

You may just win some Big Fish Motorsports schwag if you provide any info leading to the resolution of this mess.

[LJ851]

Wiring harness.

[spdracrm3]

sounds like a good candidate to run on an ignition scope (traditional oscilloscope cannot correlate each cyl pattern individually,it requires a secondary pick-up and a firing order) and look closely at the control circuit pattern (ECU controls ground side of coil) for clues as to whats going on and why . the pattern will point you to the failure ,matching control signal of each cyl to a KV output (or lack thereof)from coil .and then freeze the pattern for each cyl and study it closely for irregularity in the control circuit, dwell, voltage drop and coil saturation ,shorting....

some example patterns with descriptors https://www.picoauto.com/library/aut...rimary-voltage

do you know if its the same cylinder or cyl pair (ie 1-6 ,5-2 ,3-4) that keeps dropping out (again a ignition scope will show you this) if its the same cylinder/cylinders it would point to a possible signal failure at the same point in crank rotation (and one would see an incorrect or non-existant control signal from ECU for that cyl) then one would look very closely at crank pattern for issue (ie deviation in amplitude ,extra missing tooth in pattern ,problem in one tooth of pattern)

if issue jumps randomly between different cylinder or cylinder pairs then one would start looking for issues with interference or other sensors causing issue within ECU

[5Toes]

have you tried other plug wires? the pickup reference sensor thats attached maybe has a weird break in the wire or something...?

[ahlem]

Did you try unplugging your O2 sensor? I had a similar issue on an M30 that had a universal O2 sensor with some funky connectors. With the salt and corrosion, there was a compromised connection at one of the wires that caused a similar condition to what you mentioned. The other possibility is you have a random ground out happening somewhere. You might take a careful look at everything in the dark to see if you see an arc somewhere.
Take a chunk of dowel and move wires around while it's running and see if something happens. Safety first please, no dowels and belts may be allowed to meet.

[nando]

have you swapped the main relay? I had a similar issue where it would just cut out, I could even see it in the datalogs (must have been enough power in the capacitors to keep MS turned on). I swapped everything I could and it turned out the main relay was bad.

[JiXer]

I have tried the main relay and the o2 sensor.

[Ether-D]

Wiring harness.

Si.