How about the 5th one?
You need to check the resistance on each plug to see if they are working. Should be less than one ohm for each plug. You should also confirm that they are getting 12 volts with the ignition in the glow position. Given current temperatures in Colorado it would seem that none of them are working to cause that problem.

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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