The diesel fuel system is an extremely complex and sensitive system. Very few repairs or adjustments are possible by the owner. Any service other than that listed here should be referred to an authorized VW dealer or diesel specialist. Injection pump repair requires experience, very expensive calibration/test equipment and a large number of special tools. This type of work should be referred to a shop that specializes in diesel engine injection pump overhaul.
Any work performed to the diesel fuel injection system should be done with absolute cleanliness. Even the smallest specks of dirt will have a disastrous effect on the injection system.
The fuel in the system is also under tremendous pressure (1,700-1,850 psi), so it's not wise to loosen any lines with the engine running. Exposing your skin to the spray from the injector at working pressure can cause fuel to penetrate the skin.