95 280c misfire when hot

WE HAVE A 1995 280C WITH 6200 MILES. WE ARE THE SECOND OWNER AND KNOW THAT THE CAR WAS NEVER IN AN ACCIDENT OR ANY OTHER TYPE PROBLEMS. ANYWAY ABOUT A MONTH AGO IT RAN ROUGH SOMETIMES IN GEAR AT IDLE. NOW SOMETIMES IT WILL MISS WHILE DRIVING IT WHEN IT WARMS UP. TOOK IT TO A GOOD TECH AND HE SAID THER WAS A MISFIRE IN TWO CYLINDERS WHEN THIS HAPPENS. HE ALSO SAID THAT TWO DIFFERENT COIL PACKS CONTROLLED EACH CYLINDER. ALSO WHEN THIS OCCASSIONALLY HAPPENS YOU CAN MAKE IT RUN ALRIGHT BY TURNING OFF THE CAR. HE SEEMS TO THINK IT MIGHT BE A WIRING HARNESS. I WOULD THINK A WIRING HARNESS WOULD NOT RESET ITSELF. ANYONE WITH A SUGGESTION?

THANKS-STEVE

[suginami]

I'm the first guy that voted.

My car and yours have basically the same engine, but with different displacement - mine has 3.2 liters, yours has 2.8 liters. I experienced the same problem you are having and researched the hell out of this problem, so I can offer some good advise.

My first advise is turn off your Caps Lock button. Typing in all capitol letters is the same as yelling.

Your car is an HFM-SFI system (hot-film mass air flow sensor, fully electronic with integrated electronic ignition and sequential fuel injection), that uses coils that are mounted directly on the spark plugs, replacing the distributor at the front of the engine. Each coil pack provides spark to two spark plugs at the same time, one coil wire connected directly to one plug, and the other with a short high tension lead to the next spark plug. So there are 3 coil wires and 3 high tension lead wires.

These cars use non-resistor wires and non-resistor spark plugs just like old VW's and Porsche's. The resistance is in the "boot" or end that connects to the coil wires. Poor idleing problems are usually due to a bad connection between the coil wire and the resistor end. I would go ahead and replace each of these resistor ends. They are about $7 - $10 each.

I would also replace all of your spark plugs. Make sure that they are the original Bosch non-resistor plugs. Buy them at the dealer. Do not use resistor plugs and do not use platinum plugs. These engines run perfectly on the cheap copper core Bosch plugs.

If this doesn't cure your problem, then I would guess it is the engine wiring harness. All of the harnesses are bad from '93 - '95 on M104 engines (C280's and E320's). The wiring harness costs about $550 and takes about 3 hours of shop labor to replace.

[blackmercedes]

Thanks to Paul's excellent post, the rest of might as well bow out with any advice...

Thanks for the advice. I will pass this on to the tech and have him check these things also. I will let you know what it turns out to be.

thanks-steve