I did a quick search online for "van making black smoke" and found a wealth of advice.
One of the first is this:
https://www.bellperformance.com/blog/bid/115989/diesel-engine-problems-black-smoke-explained
One thing you could do is to get the engine fault codes read and see if there is any diagnostic info that would help.
You could even search for a forum that features your van (more searching for YOU to do) and post the question there, along with any other relevant info; fault codes and the like - but especially Make and Model, both of which are conspicuous by their absence form your original post.
zdvsba, May 2020
