After spending $1600 on my 1999 F-150 4.2L V6 Can't keep this code (P0171) from reappearing.
To date have replace both O2 sensors. MAF sensor, intake manifold, fuel filter, fuel pump and sending unit.
Any suggestions on what to focus on next?

 

Hi VinnieB, welcome to the forum. I also have a '99 4.2l, and I had the P0171 code (along with P0174, P0133, P1131 and P1132). I replaced both upstream 02 sensors as well as the fuel pump (it died on me so I had no choice) and fuel filter. No codes since. Reading your post, I have one question and one suggestion. I believe there are a total of four 02 sensors on these trucks. When you say you replaced both 02 sensors, are those the two upstream ones or some other combination? I understand it's the upstream ones that are most likely to trigger the P0171. Secondly, there is the infamous elbow behind the throttle body, part of the pcv system, that burns a hole in itself and can cause codes. I replaced that on my truck as well. It's easy to get at and not expensive. I've attached an image of the elbow. I think it's Dorman part #46022. Best of luck.

 

Per the info I can't understand if you have done any troubleshooting or just started changing parts.
Please let us know what you did to choose those specific things to change.

 

It's been at the mechanics 6 separate times and the check engine comes on after about 30 miles. He started with the easy less expensive items.
last to be installed were the O2 sensors on the left side. Thanks for the tip dontherogue I'll try anything at this point.

 

Is your mechanic using any diagnostic tools or just guessing about vacuum leaks? There are some pretty simple things that you can do to narrow it down. Freeze frame data, live data, pending codes...all clues. If the code set when hot or when cold will show up on live data. That's a major clue.

You could spend $25 on a scanner/code reader and be way ahead of the mechanic that you're using on diagnosing the source. The P0171 code won't hurt the engine. It's just telling you that something is wrong. Take some time and find someone who knows how to diagnose a lean code.

How deep did he go on your manifold replacement? He should have seen the PCV hose quality and gaskets, etc. when he was doing that.

 

I'll ask about checking the PCV connections while he had the intake off. Claims he smoke tested everything
I'm just trying to get through emissions