I know this may be tricky but I really need some advice.
104 engine, '95 C280.
Idle fine prior to 30K service.
Afterward it had lumpy idle.
Taken back 3 times now.
Plugs that were changed did not have
fouling, also has brand new wires
and fuel filter. Engine scoped for tuning.
It is O.K.
One cylinder is turbulent followed by a slight quick surge.
This is intermittant, not constant or frequent. After idling a while in drive it will get sublimely irregular.
Archives suggest:
1. wiring harness
2.HFM control unit
3. idle speed accuator (?)
4. air plate position indicator (?)
5. throttle switch (?)
My visual inspection of wiring harness
reveals no obvious degeneration.
Runs fine, smooth acceleration, no stalls.
Please offer some suggestions, I need to take it back until it is fixed.
THANK YOU. :)

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1979 300D
133K miles
1989 300SE
1995 C280

Of course the engine wiring harness can create this type of problem, & even damage out-put of the HFM control unit. Look closely at insulation on All of the engine harness. If you see any bare wires then get it replaced. Also secondary ignition failure or breakdown can cause the same symptoms

Thank you M.B.DOC
I looked over the harness again
and noticed the beginnings of
indented wear on the black
tape that insulates the harness at the
spot where it rubs the bolt head behind the
thermostat. But no wire showing as far as I can see.
I hope it is the secondary ignition
as problem source. This was my main
suspicion.
Thanks.

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1979 300D
133K miles
1989 300SE
1995 C280