For those with W123 300D, new radiators are now affordable. I got one for $130 vs used to be $350. That solved the overheating in my 1985 300D after trying many other things over years, even another used radiator first left sitting with citric acid for a week. That climb on I-5 N out of the L.A. Basin, on a >100 F day, is where I have heard new car coolant designs are verified. If you go up the grade slower, that will reduce the horsepower output of the engine, if fighting an overheating issue.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans