I had the same issue at the car wash today. Told the guy "it's a diesel, so glow the plugs before starting." He looked at me like I was crazy. "The what?" Anyway, I don't think it causes any harm, except for maybe the starter and a slight issue with overfueling before igniting. But someone with more expertise than me should weigh in.

I had heard that Daimler was considering with the Jeep Liberty diesel putting in a glow system that would be activated by the keyless entry, so the cylinders were warming without any dumb valet even knowing it. True?

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1998 E300D, 287k, barely broken in.