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.

All I can add is that as far as I know checking for a spark the old fashioned way is likely to shag the more sensitive ignition systems on modern cars. So I'd stop doing it if I were you.

Apparently a gentler way of doing it is to use a strobe light and just see if it flashes (don't look at the strobe - shine it on a dark spot somewhere under the bonnet)

__________________
1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!

How long overdue is the ignition system for maintenance? Plugs, cap, & rotor should be replaced ~30k miles and wires, leads, & coil should be replaced every 100k miles (I think this is the interval).

Just to diagnose, you should remove the cap and inspect inside. There shouldn't be any, or very little, green corrosion inside. If you have definitely traced the misfire to cyl 1 you could swap ignition wire/lead/plug #1 with cyl 4 or 5. If the misfire moves with it, you have a problem with the wire/lead/plug. It's probably time for a total ignition overhaul if that's the case.

Other common problems with the cis m104 are injectors and ignition module but first you should rule out the ignition system

Bah, I didn't read all your post. Stupid me.

One thing I'd say to try checking spark is just unplug the lead a little and leave it on top of the plug. You should hear a pretty pronounced clicking related to rpm. If there isn't one or its not very loud there's a problem there.

The ignition module on the fender is a common fault in that engine but I think that will cause a no start issue or really random misfires

Do some snooping on here for cis troubleshooting too. Injectors are fairly common, fuel distributors have been known to cause misfires too.

Yes I don't get this clicking - or see a spark at all, I think there may be a spark of some sort but if it is there it doesn't seem strong enough to jump any gap.

My question is why just on this one cylinder? Am I right in saying the signal comes straight from the ECU to the coils?

w:

Give this a try.
1) Ensure that there is correct compression in #6.
2) Ensure continuity, and non-leakage of the #6 plug wire, i.e., not grounding.
3) Ensure that #6 spark plug is in good condition, i.e., not shorted internally.
4) Ensure continuity of secondary circuit from #1 plug connector to #6 plug connector.

This is the early m104 with mechanical fuel injection, right? The 300ce in the states only came with the mechanical injection m104 that has one coil for all cylinders.

You should isolate the problem to a cylinder, fuel/air, or ignition then diagnose why. imo