Try checking for a split in the boost pipes. If there is a split, it won't be holding the pressure and the mix of fuel/air will be off resulting in overfuelling and lots of smoke. Check the one going into the inlet manifold at the right hand side of the engine at the top, near the back, it'll disappear into the wing to the intercooler. They quite often split causing excessive smoke. Could also check the one going to the turbo if that one's alright. Then it'll be a matter of checking MAF(air flow meter), EGR valve(exhaust gas re-circulation).