Audi A4 Black smoke
 

I have an 2001 Audi A4 1.9TDI. It has been emitting puffs of black smoke through the exhaust. I took it to my mechanic who did this

changed oil
changed oil filter
changed air filter
changed fuel filter

this made little improvement but then the smoke came back. My mechanic then suggested me to try and use Fuel Injector cleaners and go for motorway journeys which I have been doing for the last 2-3 tanks with just little temporary improvement. (Used STP injector clear twice and Forte Injector cleaner once)

PLEASE HELP ME AS THIS IS QUITE FRUSTRATING and I know if I take it to Audi they will rip me off as always.

Thanks for your help

Usually black smoke is due to the fuel isn't atomize ok for inyectors, try to clean the inyectors, take off the inyectors and take it to a specialist workshop.