My OM617 5 cyl will start on 2 glow plugs on a 50F morning, but takes some encouragement and it starts out very rough. I recall that 3 working glow plugs is the minimum for the dash lamp to light, so that might be M-B recommendation. I did verify that it won't start on no glow plugs, even with much cranking on a 75 F day, as I once had the GP relay box out for crash repairs and just wanted to move the car. I had to jumper 12 V to 3 glow plugs to get it to fire.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans