White smoke is an indication of either water in the fuel system (which comes out as steam after combustion) or could be incomplete carbon combustion (eg. rich fuel mix, or not enough air).
Try the basics first:
- drain the sedimentor (that's the fuel filter with the primer-pump on it)
- use a diesel cleaner additive like Redline or Amsoil or Liquid Moly in the tank to clean out the fuel lines from sludge (eg. use a bottle of liquid Moly per tank for 2-consective fill-ups)
- Check your air-filter, blow it out or wash it if it's a K&N
- Check your MAF sensor in your air-filter box, it might be dirty and sending wrong-signals to the ECU... buy a proper MAF sensor cleaner (try not to use alcohol and gasoline, the MAF sensor is VERY delicate). DOn't over-oil your K&N, sometimes the excess oil can foul the MAF sensor
Is the vehicle hard-starting? Does the glow-plug light stay on extra long?