So, I took it for a test drive. The first 15 to 20 minutes it was almost perfect, only three or four misfires in total, hardly noticeable.
After that it deteriorated rapidly and now I'm back to square one.
I'm now wondering if it's actually the spark plug cap or what else it could be.
Edit: This is what someone in the German forum wrote to my initial question:
"This sounds like voltage problems with the ignition system. The exup fault could be a side effect. An exup fault will not cause the engine to stop.
I would start by investigating if the 12V is coming clean to the ignition. A simple bulb on the 12V supply side of the ignition coils may give some initial clues.
Second important thing is the starter relay and all its little switches. This can also block the ignition. To rule that out you can temporarily replace the relay with wire jumpers.
What I have also seen are cracked plastic housings of the ignition coils. There the spark jumps directly to ground. But this can only be seen in total darkness."
After that it deteriorated rapidly and now I'm back to square one.
I'm now wondering if it's actually the spark plug cap or what else it could be.
Edit: This is what someone in the German forum wrote to my initial question:
"This sounds like voltage problems with the ignition system. The exup fault could be a side effect. An exup fault will not cause the engine to stop.
I would start by investigating if the 12V is coming clean to the ignition. A simple bulb on the 12V supply side of the ignition coils may give some initial clues.
Second important thing is the starter relay and all its little switches. This can also block the ignition. To rule that out you can temporarily replace the relay with wire jumpers.
What I have also seen are cracked plastic housings of the ignition coils. There the spark jumps directly to ground. But this can only be seen in total darkness."