Car Problem Reports
Mercedes-Benz C240 Spark plug wires causing misfire
Mercedes-Benz C240 Problem
Average Mileage: 121,866 mi (71,600 mi - 155,000 mi)
#6 occasional misfire. Comes and goes.
Visitor, 2003 Mercedes-Benz C240, 139,000 mi
I had a misfire on cyl 1 and 5. was able to diagnose #5 issue as a plug wire not fully seated onto the coil, therefore arcing and eventually deteriorating both the coil stud and the wire to a bluish dust. Replaced the #5 coil and single plug wire, problem fixed.
Cylinder 1 was a bit more challenging. I had spark, fuel, air and compression. Thought maybe the injector was dirty, so I ordered and installed a new Bosch injector... No change in misfire. Checked voltages of incoming coil wires and injector wires... All perfect. Checked resistance over plug wires... All perfect, but still a misfire. I knew it wasn't the fuel filter, but I changed it anyway because it was due. Also changed all 12 plugs with mercedes recommended NGK iridium plugs... Still no difference. I started to get concerned I might be in for some major head rebuild work!
Spoke to a shade-tree mechanic friend of mine and he said (Within 2 seconds & with full conviction I might add), "Change all of your plug wires, they're leaking electric somewhere."
Did what he said, and the car runs like new all over again!
Moral of the story, always trust the advice of a mechanic that has a small-block Chevy 350 for a mailbox at his house. (Not kidding)
Visitor, 2002 Mercedes-Benz C240, 71,600 mi
my car, bought used with 51000 miles. Had a slight miss when test driven, which got worst within 500 miles. I isolated the cylinder missing, center on drivers side. I did compression test. cylinder compression good. I change all spark plugs, all spark plug wires, and the single coil pack and the fuel injector for that cylinder. NO change. I took the car to the dealer, they did the same exact thing. completed compression check, changed all spark plugs, changed wires, and changed that cylinder coil pack, cleared the check engine light. Seem to solve the problem for about 1000 miles. Same problem returned, I don't know what to do about it.