Replace the pressure cap, too -- they go bad and allow the coolant to boil in heavy traffic or after high speed runs, causing occasional coolant loss.

Pressure test is the only real diagnostic, but the caps are cheap.

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!