M104 Bad earth? effecting cylinders 3 and 4...

[richard300]

Had a missfire develop out of the nowhere on my 1996 S280.

It seemed to move around (sometimes cylinders 1 and 6, sometimes 3 and 4 etc).... but now seems to be constant on cylinders 3 and 4.

A friend had a look and traced to fault to the connection (poor earth/permanent live) to the plug that feeds power to the coil pack in cylinder 4.

I have started removing the tape from around the loom and i am finding (so far) a loom that suffers no crumbling or degrading.

Any ideas? Is it worth trying to locate this earth fault or would this point to issues elsewhere that i should explore?

 

[grober]

Pelican Technical Article - Mercedes-Benz W124 - Spark Plugs Coils and Wire Replacement

In essence the coil [ powered with ignition on] is fired by earthing each coil through the ignition transistors in the ECU. The system is lost spark meaning 1+6 fire together 2+5 and 3+4.

The coil has an extender under it to reach the plug it sits on- these can go faulty. Replacement is best option. Coils can go faulty- best replaced ASAP as they can knock out their corresponding ignition transistor in the ECU which is expensive unless you can strip it down and repair yourself.
There's a DIY on here somewhere.

 

[richard300]

Changed all the plugs, leads and coils and moved them all around to eliminate a 'duff' new part.