Do you have a VW diesel that used to be powerful and now it's weak? Did your fuel economy sink from 50 to 40 or even less? The problem could be carbon clogging the intake. Follow me on a pictorial tour of this diesel performance problem.
Here is a "normal" intake with no clogging. Basically, it's a clear piece of cast pipe . . .
After 100,000 miles that pips can get pretty plugged up from carbon, a byproduct of diesel combustion . . .
Here's another view through the throttle body. You can see that half the pipe's capacity has been lost.
When the pipe clogs up you have to push the throttle farther to get the same amount of air into the engine. It becomes like a person with clogged arteries or asthma - a bad situation. It does not take sophisticated mechanical knowledge to understand what's going on here.
The cure is to remove the carbon. Sometimes you can do the job with a flexible wire brush and a shop vacuum. Other times it's necessary to remove all the intake plumbing and ship it off to be boiled in a chemical cleaner. It can be an expensive repair. Once it's done, your fuel mileage and power will rise back toward their previous levels.
At the same time, it's always a good idea to change your fuel filter. They clog too, but the clogging is inside where you can't see it. I suggest changing your diesel fuel filter every 30,000 miles.