94 C280 Spark Plug Foul
 

Replaced the spark plugs 3000 miles ago. I started having some backfire occur; so I cleaned the MAF and Air temp sensor and found the rough idle cleared. I decided to order a new MAF and Air temp sensor and left the car sit until I replaced them. Car sounded great sitting at idle. Once I headed down the road, it started to backfire and lag.

I pulled the plugs and the #2 cylinder plug was fouled and black. I replaced the plug and took the car out for a 5 mile spin. As the car warmed up things sounded as it should. As I came back into the subdivision the car backfired a few times.

Since only one plug is fouled, I would suspect that I may have a leaking injector. Any thoughts?

Brian

It could be a leaking injector or possibly the engine harness. The injectors are held "hot" and are grounded by the module. If the harness is deteriorating and that injector lead has found another ground, then it could be held open. Try taking a quick inspection of the harness and if there is no cracking on any visable wiring then swap the injector with another and see if the problem follows the injector. It's a lot of work for a diagnosis, but just throwing parts at it gets expensive.

Thanks to A Dalton, I tried his suggestion first. Apparently the spring clip in the coil resistor boot wears out and should be replaced each time the plugs are replaced. Well, I replaced them and the other three wires. So for now the Mercedes is back on the road without backfire on my 5 mile trip. Hopefully it continues to roll without incident!

Brian