Dear customer, I understand the problem you present first Let me explain:
Black smoke is nothing but vaporized coal: when the fuel is hot and does not have enough oxygen for combustion, there is an erroneous explosion with a lot of traces of fuel in the exhaust which causes the smoke you can see of dark gray coloring.
This means that your vehicle is either not giving enough air to the system because of some problem in the turbo, or the injection system has some problems and is injecting much more fuel than air. I want to clarify that the mixture of air and fuel inside the cylinder must be stoichiometric, this means that there is equal amount of air with equal amount of fuel inside the cylinders.
As diesel is basically oil, leaks or clogged injectors cause the engine to inject more fuel than necessary, and therefore smoke is produced.
This type of problem commonly happens due to lack of maintenance and partnerships in the system. I always recommend to first perform a computer diagnosis because this may indicate the error codes that the computer may be generating. If there comes some defective component that detects a sensor, for example if you have problems in the mixture, it is very likely the exhaust system will detect it, or if you have a problem with the advance of the injection pump can also be observed in the error code of the fuel regulator that detects a high pressure.
In short dear customer, if the problem does not detect a sensor you will be able to attack the problem more accurately and with a reduction in costs. Therefore, i always recommend first make a computer diagnosis in these jobs.
Dear customer, of course sometimes the scanner may not indicate anything and this is because many times the consumptions are produced by lack of maintenance. So, if this is the case what you are going to do is first replace all the filters, you must perform a maintenance to the injectors and the injection pump, do not use additives to clean the fuel system on the contrary you can cause greater damage, finally it is not too much to perform a review of the air intake system, the throttle body and the turbo and its pipes, because as I explained, it can not only be a problem in fuel injection but if there is little air in the system gives the same result with the smoke in the exhaust.