I've got the 617.91 manual and nowhere in there does it say that the glow plug timer varies according to temperature. It says that the light does, but that if the engine is not started, the plugs will continue glowing for up to 90+20 seconds.

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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