2006 TD4 running issues, injector?

Good evening

My mate has a 2006 TD4 and it has developed an issue when driving. He states that when he attempts to accelerate, it is hesitant, and makes a knocking type of noise, and not a lot of oomph. There has been some black smoke out the back, which he assures me is not a common occurrence for his car. It will start ok, and will idle ok, however, when he revs it from idle, it will make the same kind of noise. He has had both fuel pumps replaced not so long ago. He is thinking along the lines that one of the injectors may be faulty, but I am not so sure. He hasn't noticed any significant increase in the use of fuel, nor any leaks. He told me he has checked the pipes leading from the intercooler etc, and thinks they are ok. I have leant him my code reader to see if that sheds any light, however, I thought I would ask on here too incase anyone has any ideas? Cheers all

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