in my search on this issue, everyone seems to be saying they run at 80c, no matter what the ambient temp, driving conditions, etc. -- i just went for a drive in 95f, ac on, first on the flats, gauge stayed at 90c -- then took it up hill, climbing 1000' in 7 miles, doing about 50mph average -- temp went up to 110c -- this dropped 15 degrees if i put the heater on.
new temp sending unit, and thermostat, plus i checked the temp gauge by using an ifrared thermometer at the engine, and the gauge is correct.
time for a new radiator? -- it looks fine to me, but this is an 81 240d with `180k on it.

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81 240D: Lola
77 toyota chinook: Carlito
93 chevy k1500: Cowboy