i suggest reading the mercedes owners manual. in cold conditions pump the pedal 3 times and then floor it (let off when the engine catches of course) and keep the starter turning until the engine is idleing smoothly meaning dont stop cranking when you hear one cylinder going KEEP GOING and remember on the first try you every try to start a diesel will be your BEST CHANCE after that you are coating the precomb chamber with cold diesel remember dont ever EVER use any starting fluid and if you for gods sakes have to use some type of starting fluid use wd-40 and make sure to put the fluid it WHILE THE ENGINE IS CRANKING NOT WHILE YOUR PREGLOWING. it sounds wrong that you have to crank it for so long mabie your glowplugs are shot maybie your relay is gone i would test both of them make sure they are both good. you should see the "saltshaker" light up when you turn the key to position 2. also mercedes says you can crank the starter for up to 30 seconds i would do it for maybie 15 but any more than that your risking heat damage.

Also if you havent adjusted the valve lately its a good idea if they are "tight" this will cause a loss of compression making it hard to start.

that is all...