A couple of my plugs did have some carbon on them. Not a real heavy covering but maybe enough to hurt the heating. Maybe that's what made the difference. I wonder if these should be cleaned up once in a while. I recently did my periodic "Italian Tuneup" so what carbon was on there stayed on through that.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold