Since I've had the car, the Dump Valve doesnt always *ahem* dump.

It will not go off whilst the engine is warming up (to a normal temperature). Nor will it go off if I have been driving for a good 30mins (car warmed up) and the passing air has cooled the engine bay down.

Sometimes it goes off (not as loud) when the turbo fully spools up, but not always.

Anyone have any ideas ?