Usually a bad visco clutch, specially if it gets warm (100C) in slow traffic.

Otherwise, most likely a bad thermostat. Third choice is a bad pressure cap, fouth would be bad water pump, tying with a leaking hose.

Temp WILL climb on heavy load (long upgrades in very hot weather in the mountains), but with a good visco clutch and thermostat, my 300D 3.0L never budges from 82-85 no matter what the driving condtions. Ditto for the TE now.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!