Help with a TDI

Hello, I have been here for a long time but it is the first time I write, I tell you a probmon I have if you can help me, I am the owner of a Seat Cordoba 110cv tdi of the 98 type engine AFN and has loss of power in low and in the workshop they do not hit Haberia and then I and a friend who has another the same car, and our surprise is that now yours does not throw down either, what could it happen? We have changed all the empty tubes and it remains the same, what I have observed is that with much that I stomach not made or drop of smoke as if it were lacking food or there was something that limited the injection, Puuuuuuuuuuf I am desperate and I do not know what to look.