Bad injector or seal causing lean running will do this.

Combine that with low grade fuel, and you get detonation on the highway (that isn't audible, BTW), and melted pistons, burned valves, etc.

Good reason to check the plugs once in a while, and investigate running problems funny noises pronto.

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!