An MB diesel should start with no block heater above 0F. Below that you'll need a block heater. MB recommends 4 hours for 0 - 32F and all night for below 0F.

Check list (if I've missed something anybody is WELCOME to add their two cents worth):

Valves MUST be properly adjusted (must not be even a tad "tight" and not grossly loose).

Glow plugs must all be working optimally and must be energized for a sufficiently long time to properly heat the prechambers (this may require glowing 10-20 seconds AFTER the dash light extinguishes - especially if there is carbon accumulation in the prechamber or the plugs have started to fatigue).

Battery MUST be fully charged and have sufficiently low internal resistance to deliver the needed glow plug and starter current. Conventional lead-acid batteries more then 5 years old are seldom reliable (and many fail sooner, though that has NOT been my experience with Interstate and Johnson Controls manufactured Die Hard batteries). Batteries that have been maintained exceptionally carefully and special construction batteries like the Optima MAY last longer, but I have a lot less experience with that then with conventional lead/acid batteries maintained rather normally!

Fuel must be properly treated for temperature to minimize clouding and be free of water. Filters must have full and not marginal flow. The fuel system must be free of any leaks (and this includes the hand pump - if it leaks when you pump it - it MUST be replaced).

Oil must not restrict cranking speed (either a "lighter" weight conventional oil or synthetic is HIGHLY desirable for prompt starting).

After an appropriate preglow period, press the accelerator pedal to the floor (to move the rack away from the "stop" position) and as you start cranking, release the pedal to about 1/2-1/3 and do not stop cranking until the engine starts and is running evenly! You may only get one shot at this and you need to play ALL of your best cards the very first time.

Marshall Booth