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

Did he bleed the air out of the system after he refilled it? the early w123 have a strange hose setup so the top cannot self bleed, there is a bolt that is suppose to be loosened to remove the air....if the bolt isn't froze to the thermostat housing....in that case you need to put water into the upper radiator hose, then reconnect....he may also have a stuck thermostat, the evil servo could have popped causing loss of anti freeze too.

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Should have a bleed screw on the t'stat housing.

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83 SD

84 CD

I don't believe he bled the air out. It's hasn't overheated since, but still idles at the 212°F line. Fan clutch appears to be good.

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