Essentially when they won't work properly. They've been known to run 250,000 miles without replacement on well maintained vehicles.

What usually happens is that either the opening pressure drops too low or the orifice wears to the point it won't spray properly, giving a solid stream or off-center mist. Can only be determined by removing and testing. Both conditions cause excessive knock.

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!