Hello all...
Here is a summary for y'all regarding DTC P0171 and P0174 on my 1996 F-150, 4X2, 4.9L inline 6, automatic trans, with A/C. Info only, no replies necessary, but feel free to if you want.
P0171 System too Lean (Bank 1)
P0174 System too Lean (Bank 2)
Check engine light came on, fortunately I have a handheld scanner.
Looked on this forum site for the two DTCs from my scanner and found that it could be a lot of different things, vacuum leak somewhere, MAF, Fuel filter, etc.
Checked for vacuum leak on all visible hoses--nothing seen
Cleaned MAF (isopropyl alcohol and q-tip--very carefully)--DTCs still present
Replaced Fuel Filter (which was plugged up pretty good)--DTCs still present
So, not wanting to shotgun troubleshoot and buy/replace parts that weren't bad, I took it to the local Ford dealer (gasp!)
I wanted them to pinpoint the problem and let me know what it was in case I could fix it.
They called and said my upper and lower intake manifold gaskets were leaking allowing unmetered air into the system thus causing the DTCs.
Well, I had them replace the gaskets ($30 parts, $400 labor).
No more check engine light.
I wanted to post this summary because I didn't see any posts for the two DTCs with intake manifold gasket leaking as a possible cause--maybe I didn't search enough?

The end.