I've been suffering low power above 3k rpm, power would come back after changing the filter, but recently it's better but not up to full power (for a 300tdi that is).

Today I cleaned out the sedimenter and checked the fuel sender, all ok.
So I left the return pipe off the sender and started the engine, the returning fuel is full of air bubbles.

There's no air going into the pump but there is at the outlet.

I've had this with Peugeot's in the past and it's been the injection pump shaft seal.
But from what I've read on the 300dti fuel comes out the seal into the timing case not air pulled in.

I've opened the banjo bolt on the filter outlet, there's fuel pulsing out when cranking the engine, so I assume that the lift pump is working.

Can anyone shed some light on this ?

Anyone.

Leak off pipes perished on injectors

I think it may be caused by cheap filters and bio.

Probably a faulty injector passing compression gasses into the leak off pipe