If the thermostat only opens when the water is near boiling, it's bad.

You need to check the temp -- probably should start to open at 80 or 82C, and be fully open by 85C.

A badly clogged radiator will cause the engine to overheat even if the thermostat is good.

I'd use a thermometer and check the temp of the coolant with the radiator cap off to verify the temp -- guages and senders have been known to be wrong!

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!