Spent several hours today working the truck over.
Checked Plugs/Coil/Wire sparks, generally good on all.
Replaced MAF sensor, good readout on scanner tool
Cleaned fuel rails and injectors, both banks
Checked Compression, fine
Checked fuel pressure, best I have seen
Vacuum hoses seem to be good, a sucking is present.
FPR has no fuel around hoses, and all fittings are clean.
Found 1 cracked plug, replaced, no fix.
Ran Seafoam through rails and tank, no better
Started to replace Intake Gaskets, but didn't see signs of wear or gas leakage around seals from front of intake to head.
To recap, the truck is showing P0171 and P0174 codes, and missing under mid/high throttle when under a strain. No 300 code, or cylinder missfire codes have been thrown yet.
Truck also sounds like it's backfiring in high RPM's under the hood, probably from the intake side.
I am really not sure where else to look at.
Has anyone had similar issues on a 99 5.3 Vortec, or have any idea why i am getting lean codes on both banks? Is there another sensor I need to test/replace. Read that the Knock sensors and coolant sensors could help this lean situation, but posts are on older models.