If you could push it up the hill to start with, the hill was probably not steep enough to get enough speed to turn over the engine when you let the clutch out. (I assume we're talking a manual transmission here). Typically you roll start in either second or first. Without glow plugs operating, you'll need to turn over the engine more than normal to get it started, hence the need for a steeper longer hill.

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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