1991 Mercedes 300D Turbo Diesel - HELP!

Please help me. I have a 1991 MB 300D 2.5L Turbo Diesel. When I attempt to start the car, it has trouble staying in idle. I have to pump the gas pedal 4-5 times in oder to get the engine to run and have the gas pedal down at least half way to keep the engine running. As the engine runs, there is a cloud of white smoke coming from the exhaust. There is no blow-by, however when I uncap the radiator cap, a froth of bubbles rise after 5 minutes or so of running. I have some ideas of what it is: cracked head, something in the fuel system is not delivering enough fuel, (but will this also cause white smoke?). Please if any one has gone through this situation, please help me out. I am in the market of looking for a used engine also. Thanks.

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