02 Audi a4 losing power

You have blown a turbo charge pipe somewhere, check all hoses...Once this happens the car will begin to run very rich, as evidenced by the black smoke. Do not continue to drive like this, as the raw unburned fuel in the exhaust will clog the catalysts and that can be very expensive.

It isnt piping, The external controller for your internal wastegate is blown, Since the boost can escape or isnt being held, your car should be in safe mode, usually sounds like turbo is starting to spool then stops, There isnt supposed to be an aftermarket diaphram available for the K03 but i hvae seen them on ebay. If the wastegate itself is still intact then replacing the controller and clearing the maf/o2 codes along with that should get you out of safe mose and solve the problem. If u can find an aftermarket wastegate controller, one from a 300zx works and improves response a bit.