My 94 C280 started to have a rough idle at stop lights. After a couple of days of this behavior my "check engine" light came on and my car started to behave like it wasn't running on all cylinders (rough idle, constant shaking, and couldn't go over 55mph).

I took my car to a shop and was told I needed an Ignition Coil Harness (Engine Harness). I bought the part myself, for over $700, and installed it. A lot of the wires looked pretty bad so I was sure this would fix my problem.

After installing the new harness I started up my car and took a test drive...everything seemed fine but after driving for about ten minutes the problems returned. It seems that braking provokes the problem. A couple of times I was able to rev the engine in neutral and the problem went away for a short period. I pulled the spark plugs and they looked ok (although I wasn't able to get one of the six out).

Thanks in advance for any advice.

Jason

 

I've got the same car. Just replaced my wiring harness too. Same problem.

Car was idling rough upon starting, but had plenty of power. It stranded me (burned out OVP) and I found oil in the spark plug wells from the leaking cam cover gasket. I cleaned out the oil but in doing so I damaged the original wiring harness. Car ran worse.

So I forked over $700, changed the cam cover gasket while I was at it, new plug wires and boots. Idle surges between 500 and 1200 rpm now. Car isn't drivable. It won't sit still at stop lights.

Did you ever get a solution to your problem? I'm going to have another look at my vacuum hoses. Maybe I knocked one loose. I also seem to remember that a bad wiring harness can damage a throttle body actuator. Keeping my fingers crossed it's something easy.

 

Got a new coil and tried it in each position. No change. Still got a lot of soot coming out the tailpipe. Same trouble code of P0300 "Multiple random cylinder misfire".

 

It's fixed. Bad MAF.

In the process of chasing the problem here's what I've learned.
1) My independent mechanic mis-diagnosed the problem as a bad throttle body and/or engine computer (ECU)
2) You can rewire a throttle body if you have a soldering iron, an ohmeter and a lot of patience.
3) The copper core spark plugs Bosch F8DC4 didn't make any difference in the way the car runs.
4) You can test a MAF (Mass Airflow Meter, Mercedes calls it a Air Mass Meter AMM) with a 9v battery, hairdryer and a voltmeter. Analog voltmeter is nice, you can watch the voltage change nicely. Bosch has info on AMM on the web showing output voltage.
5) A bad OVP will give you an "Error" code on your cheap wal-mart code reader.