Hi,
Newbie here on forum.

I am trying to diagnose 2005 LLY problem.
Truck is all stock with 115k owned since new.
Had my issues with broken glow plugs(by dealer) with a 7K price tag.
Water pump been replaced 3 times by dealer (in a year)
A ground problem were wire corroded inside insulation. Replaced all engine grounds up to harness on top of engine with marine wire and terminals.
NOW: Started sounding different and did have low fuel rail pressure code.
Truck has constantly changing fuel rail pressures (sometimes below recommended).
Trucks does maybe 3k per year now and I change fuel filter once a year.
Found air in fuel line by installing clear hoses.
Replaced complete filter assembly with GM part.
I added Airdog II lift pump/filter.
truck runs better however, not as it used to and sounds different.
Installed vacuum/pressure gauge on factory port and observed 8 PSI with lift pump on. Disabled lift pump and had 2 in hg. Disconnected fuel line just before port and ran clear hose to diesel can. Engine will run however still only 2 in hg and line never completely fills with fuel. Obstruct line and I can get 12+ in hg.

Is my CP3 Failing?

Sorry for long post.