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Your GP relay may be getting tired -- the time the GPs stay on is determined by a temp sensor on the relay, so if it is going bad, they won't stay on long enough. New plugs won't help.

The contacts can also get burned, so you don't have enough current to get the plugs hot.

A new relay isn't that expensive. I'm fairly sure you don't have any other problems.

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!