Backfire is fuel igniting in the exhaust, could be ignition related (when were the cap, rotor, and wires replaced, if ever?) or serious over-rich running followed by normal mixture.

The latter could be mass air flow sensor, temp sensors, or stuck fuel injectors.

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!