Well my GP's did work correctly but were very slow and didn't seem to have the heat of my new ones plus if one fails of my new GPs, the car will still start. This caused problems in the past when one shorted out on my wife and she was one hour away. I like the comfort of not worring about that so much I pulled out perfectly good plugs and replaced them.

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1977 300D Lost coolant while someone else was driving
1983 300D Can't run without oil
1985 300SD (gone but not forgotten)
1990 300TE 4matic Sold
1991 Yamaha Venture
1975 Kawsaki 250 triple
1974 Honda 200CL
1951 8N Ford
2008 Wildfire 650C