My new (to me) 1988 300CE was idling a bit rough, so I changed out the plugs and ignition wires. The car has 70K miles on it, and the wires were obviously original. The plugs I pulled out were Champions?. Installed new Beru wires, but could not readliy find the recommended Bosch copper core H8DC plugs locally, so I put in a set of NGK substitutes. It ran decently for a 100 miles or so, then the rough idle came back. Pulled the plugs again, all were black and sooty, especially #s 1 and 3, which were oily and fouled looking. I picked up a set of slightly hotter Bosch H9 DC 0 copper cores, and put them this morning. The H9 DC 0 seems to be the plug of choice on this forum. My question is, should I be worried about the condition of plugs 1 & 3 at this point, or just run them awhile and see what happens? Is this fairly common with these cars? Thanks for the great advice as always. Don

 

The oily plugs is the tell tale signs of neeeding new valve stem seals, or the more costly route, a valve job. Just run the H9DCO for a while and then pull them. If they have the oily coating, then you know what is needed. Valve stem seal replacement is about $150-$250, and the valve job will run you $1000.