Dump valve won't work on a JTD, cause there is no throttle to close. On a turbo petrol, coming off the accelerator closes the throttle butterfly, allowing a build up of pressure outside the inlet manifold for the valve to then 'dump' in a big hiss.

On a JTD (or any TDi) there is no throttle butterfly, so any boost the turbo creates when you come off the acclerator just goes straight through the engine, only no fuel is injected.

I think this might have been one of the first things I asked when I bought my JTD