Taking the thermostat out unsually causes serious overheating. If you get BETTER cooling, it's time for a new radiator, the flow is way down. At least a tank and clean (but if aluminum, toss it).

Cooling fan will always turn slower than crank speed, even if you are going 70 mph, since it is designed to PULL air across the rad even going that fast. After all, you're spinning, what, 3200 rpm? and the fan pulley is smaller than the crank pulley.

The heat comes from the rad -- I can hear the fan on the 300D and the 280 when the rad heats up, ditto for the Volvo (that one sounds like a jet engine).

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!