Unplug the connector from the GP relay to the plugs and check the resistance between the connectors inside and ground. Must be 0.6 to 0.8 phm -- either higher or lower is a bad plug. Most likely infinite (broken filament) or grounded (0 ohms).

Replace bad ones, or just replace the whole set and don't worry about which one is bad.

This engine is known for flat "lifters" -- make sure the o-rings on the center shaft of the oil filter holder are good, since they will allow the oil to drain out of the lifter gallery if not (down at the bottom, probably look and feel like plastic by now).

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!