Without a meter or a test light to see if you have voltage at the plugs, determining precisely what is wrong is very difficult. Testing the resistance of each plug is the best way. Cheapest thing to do is to assume it is one or more bad plugs so replace them all.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13