Alright I'm completely stuck. I bought my truck this past summer with the knowledge of a possible fuel pump issue. I replaced the pump assembly and fuel filter along with the plugs and wires and then put some seafoam in and cleaned the maf sensor as well. The previous owner said he replaced the o2 sensor, but it was his second truck and he didn't wanna spend time/money trying to fix it. The p0171 code has been a constant but after replacing the pump it was running a lot better. Recently it's been struggling to hit highway speeds particularly going up hill(55/60 is usually about all I get on a good incline). Took it to the shop and was told it could be related to the straight pipes and not getting back pressure or that the maf sensor could be bad. It's a small shop and they try to save people money but I really would like to get to the root of the issue without throwing a ton of parts at it. I did change the pcv valve which was pretty gunked up and that made the check eng lt turn off. But the light comes on intermittently so I'm not sure how directly it affected it. At this point I'm stuck and since I live in Vermont, an arctic wasteland this time of year, and don't have access to a garage I have a limited amount of backyard repairs I can do. Any help/suggestions would be greatly appreciated. Also, this is the 4.0 v6 eng.

 

Welcome to FTE.
The code Numbers you've posted on your 99 4.0L say that the computer is Very unhappy with the bank-1/passenger side, upsteam/before cat converter O2 sensor.
That its output voltage switching range is slow, or its not switching/responding at all & bank-1 is running lean/too much air.
This can fry the plugs, valves, pinton crowns, oil, as that bank running lean will also run Hot, so this problem needs to be put right without delay.
Normally the upstream/before cat converter O2 sensors are ready for replacement between 75-100K miles & Both should be replaced at the same time, with a like design O2 sensor, so to keep both cyl banks fuel trim in balance.
Disconnect the battery B- cable before replacing, so that the corrupt fuel trim tables in the KAM will be erased & the computer can begin to rebuild the fuel trim tables with input from the new O2 sensors.
This also wipes the cold & warm idle strategy, so if you have an auto tranny, you'll want to take the time to perform the idle relearn procedure here, Ford Truck Enthusiasts Forums - View Single Post - high idle problem to prevent possible driveability problems.
Some thoughts for consideration, let us know how your trouble shoot goes.