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.
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.
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