P0171 Only When Offroad / 4WD

Alright, still stumped. 3.4L engine (5vz-fe). At first I thought it was my snorkel causing it but it wasn't. I've narrowed it down to whenever I'm driving off-road in 4Lo, the truck throws a hissy fit and runs lean. City driving and highway driving have absolutely no effect and do not cause the truck to run lean.

List of things I've checked/done:

- Both cats, replaced the downstream cat.
- All airlines are tight and secured, physically checked as well as using propane torch.
- MAF sensor cleaned and checked for readings, A-OK
- A/F sensor checked using FSM method, behaves as normal.
- Downstream O2 throws P0136, but this does not attribute to A/F ratios.
- Reset ECU by unplugging battery
- Seafoamed engine

I hate to add yet another P0171 thread to the forums, but I simply cannot figure out why the truck only does this when off-road/in 4Lo. Has anyone else had this issue? Is there some type of compensation the engine has to do when running in 4-lo that mine is neglecting? Any input is much appreciated. Thanks all!

 

That's weird that it only does it in 4lo, really weird. I had that code and it turned out to be a vacuum leak on the back of my supercharger. It's almost always a vacuum leak that causes this code (according to the mechanic that fixed mine) or an MAF sensor issue, but that would cause it to be on all the time not just when in 4lo.

Subbed for outcome...

 

Right? Mine is N/A. I had had the hose for the fuel pressure regulator plugged with an allen screw for the longest time (4-5 months or so, forgot about it, oops) and it didn't throw a code. Re-connected it, didnt make a difference. When I seafoamed it it ran like a top for a week until I went wheeling again and it threw 171. It used to run lean on long highway trips but doesn't anymore.