White smoke fron transit van?

hi i have the same problem as an earlier question, i have white smoke from my transit van 2500. i do not have oil in the water or water in the oil and don't have a knocking sound, the smoke seems to clear a lot when in 4th gear , the van is a diesel and is only used on short runs and stopping and starting a lot as its an ice cream van, i didn't know if it needed a long run to clear it as i have never had a diesel before. i hope someone can help. thank you