u/Far-Fix364

I’m after some advice on a VW T4. My van is a 1.9td engine and when I put my foot down there is no power and black smoke shoots out the exhaust. From what I’ve seen online it looks to be a rich air/fuel mixture, I’ve done the easy things like changing the fuel and air filter but no change. It’s been at a garage for 3 weeks and they’ve not been able to source the issue. Is there anyone who has had this issue before and could possibly advise on what to do?