2.5 diesel engine blowing blue smoke when I accelerate?

I have a ford transit 2.5 diesel van 1993, I`ve just had the engine rebuilt as it lost power and was smokeing terrible. The van now has power but as soon as I accelerate I am getting a burning oil smell and excessive blue smoke from the exhaust. I took it back to the garage where I had the work done and they said it was the crap that had built up in the exhaust and it would eventually burn itself out. 3 weeks on and the smoke is as bad as ever. I use the van 7 days a week as it is an ice cream van. Anyone got any suggestions help please.