I keep getting the code PO174 for My'99 f150 , 4.2 v6 motor.

did u have a smoke tst done? need to read fuel trim with scanner. these are known for intake manifold leaks and intake runner leaks. you can test msss are flow with a test but only for grds and signal with volt meter

Agree with 1 the pcv hose on rear of intake i bet is the trouble. Hard to even see, just fixed one I pulled the intake to do the work but there was other repairs i needed to do. It may be accessable otherwise! Intake leaks are common too. Same code in this one and this fixed that also, the hose turns into mush and then leaks vacuum.