arcs 'n' sparcs, wasn't WGS or ANH was it?
Also seem to remember that a leaking or blocked upper deck line can cause similar issues. I think that UDP is an input into the fuel pump and is also used as an ambient reference for the direct reading fuel flow (read pressure as arcs'n'sparcs pointed out)...
I got quite good at setting up all the pressures with the 'blue box' on the GTSIOs in 404s and 421s in my day. Sadly those memories have given way to MECs, HMUs, EECs, CDU interrogation and seat recline mechanisms.
'ave a good night!