I hate to say it, but I don’t think the boost is the issue at all. If you’re losing fuel pressure, you’re losing fuel going into engine, killing the power.. the engine can’t make boost if it isn’t getting fuel. I think the boost drop is a by product of no fuel.

So I guess need to sort the fuel pressure drop.. maybe that regulator? Or high pressure fuel pump dying?

__________________
2004 X5 3.0d
2010 X5 40d

New Zealand