Don't drive it without the thermostat in place, it will overheat.

Replace the fan clutch, flush and replace the coolant with MB of Zerex G-05 ONLY -- the green stuff will eat the head, sure as the sun comes up, even if changed often.

While you have the coolant out, pull the radiator and clean all the squashed bugs and road crud of it, they fill up pretty fast if you live in an area with lots of flying insects in the summer.

Oh, and replace the radiator cap while you're at it.

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!