Posting for my friend with a 1978 300D. He says the temperature went way up and steam came whistling out -- it was at 225°F when stopped but 175°F while driving. He pulled into a gas station and refilled the radiator with hose water, still idling at 225°F. I'm guessing the water pump is starting to go but haven't dealt with major cooling issues.

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1985 190D 2.2L 717.411 5-speed
1983 300D Euro OM617.950