Bmw dream shattered

I have problem with my BMW 320D i have tried on several times to tell BMW Worcester that tere is a problem with the vehicle sold me I also contacted BMW South Africa aswell as B M W Germany but my efforts was invain. I haven't receive any feedback from Germany and BMW South Africa excuses is that there is a problem with the type of Diesel I used and they also blame the outlet system of the car for the black smoke at the rear bumper.. I hope someone can assist me with my problem cause they also told me that its because my vehicle kilos that it is smoking.