I think one bad one will cause a problem on a 79. I think the OE on that year are loop plugs. How have you confirmed that the glowplugs are working correctly? You could just glow it until you heard the relay time out, turn off the key and then turn on and glow again for a double length glow without changing anything.

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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