Sounds more like a glow plug issue. Does your glow plug light come on when you turn the key to on? If it does come on, are you waiting for it to go out before cranking it over?

I would check and test glow plugs first. Look here for a good how to... Mercedes Diesel Glow Plug Repair

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2012 Mercedes ML350 Bluetec 102K (hers)
2005 Corvette 55K (fun car)
2002 VW Jetta TDI 238K (mine)
1998 Volvo S70 T5 Turbo 211K (kids)
1994 Ford F150 4WD 246K (firewood hauler)
1983 Mercedes 300D 384K (diesel commuter)