04 S40 fuel pump pressure figures?

[Speedytom]

I'm having a bit of bother with a persistent po171 code on my phase 2 S40 1.8.
My investigation has led me to doing a fuel pressure test on the car.

Fuel pressure at the rail is 40 psi on startup, dropping to 35 psi at idle. When I rev the car it drops by 5 psi, which I'm pretty sure is weak, but I can't find any figures online.

The fuel pressure regulator has tested out fine.

I looked at the values on a new pump, which states 52 psi pressure. What I don't know, is whether that pressure would carry through to the rail.

The car drives fine, apart from what I would describe as a slightly lumpy idle, but nothing I would worry about unless I had the engine management light on. Because of this, it leads me to doubt my diagnosis of a weak pump.

I could get a secondhand pump to see if it made any difference, but what's to say that would be any better than mine!

Any help would be greatly appreciated!