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

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93 190E Sportline (RIP) (45K)
92 190E (Xferred most Sportline parts to this body)
88 300CE (71K)

And the Jeeps:
01 2WD Cherokee XJ (35K, Mama's grocery/kid hauler)
91 4WD Comanche (300HP stroker)

I'd be concerned and would keep an eye on those two cylinders.. The black and sooty would lead me to think you have bad injectors in that cylinder or you were not getting enough juice to the spark plug leading to incompete combusion.

When was the last time your cap and rotor was changed? M103's eat caps/rotors far faster then they do wires.

Jonathan

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Blue Ridge Mercedes
Jonathan Hodgman
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Enthusiast Service, Restoration & Tuning.
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Located in the Atlanta area
Specializing in all pre and post merger AMG's including Hammers and DOHC M117 engines.
Mercedes Repair Atlanta

My plugs came out black and sooty until I replaced the O2 sensor. Do a search on mixture adjustment and O2 sensor test. If your running rich and your O2 sensor is working properly the EHA will try to lean it out, but if it is mechanicaly adjusted way out of whack then the EHA can't compensate. If you are running rich and the EHA readings suggest that everything is OK then I would suspect an O2 sensor that while it may still be operating has slipped out of adjustment.

Jorg

Well, it's only been one trip back and forth to work (about 80 miles), but what a very noticible difference. No rough idle whatsoever, a very smooth 600-700 RPM stopped in gear and about 1000 RPM in neutral. Felt like more power too going up the mountain coming home, no downshift as before. The NGK's were TR5's, which is a couple of heat ranges down from the Bosch. Did not know these engines liked the Bosch copper cores so much - I'm convinced. Will keep an eye on cylinders 1 and 3, hopefully the only problem was the plugs. Thanks for the replies. Don

__________________
93 190E Sportline (RIP) (45K)
92 190E (Xferred most Sportline parts to this body)
88 300CE (71K)

And the Jeeps:
01 2WD Cherokee XJ (35K, Mama's grocery/kid hauler)
91 4WD Comanche (300HP stroker)