1993 320 6 Cyl Running on 5

[weisdaclick]

This problem has just started:

1993 W124 320CE - UK car with catalyst M104 engine 145k miles

The engine is not smooth any more and coughs when accelerating, however if I rev to 3,000 at a standstill it seems pretty smooth so it seems under even a light load there is a fault. I have the 3 coil ignition system.

I have noticed that cylinder #1 (front of the car), is not firing properly, or perhaps not at all. If I pull the plug lead off that cylinder while the engine is running there is little change if any to the idle, if I ground that lead onto the block there is no spark whereas the others emit a nice spark.

I have a few spare parts so have changed the coil - fed from #6 cylinder, I also have a spare ECU, I changed that, I have also changed the plug positions of that cylinder. None of these make any difference, I can't change the lead as it is the longest one, but I tried that lead on another cylinder and it was sparking fine.

So now I am thinking the only thing it can be is the low-tension wiring from the ECU but would that not also affect #6 cylinder? The wiring is pretty brittle - usual W124 fault, and has been partially replaced many years ago.

All thoughts and comments welcome.