Two approaches:

If you think they are sick, take them out and have them tested.

Otherwise, 250,000 miles or more is normal, if the pump isn't damaged and the filters were changed properly. Nozzles aren't usually a problem until you really pile the miles on.

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!