O2, Fuel trim and nearly useless motor
 

07 5.4, just bought truck and it's not licensed yet so I can't road test much but...
Truck originally had a miss, #6, found a cracked plug (new!) and replaced it. idle on all 8 seemed decent and responsive to throttle so didn't go further. Had a gross leak on passenger exhaust manifold so that was next, horrible job! 3 broken studs.

Anyway, after that was done, test drove around the block, you can lay rubber in first 10 seconds then builds a stumble/miss to the point of back firing through intake etc. Since 3 cops/plugs had been done before I bought it, did the other 5, couple scary ones but all came out. This netted ziltch, no change in road test.

Ran my scan software and that brought something to light! Fuel trim on bank 2, floating +/- a couple percent and O2 fluctuating constantly from .2v to .6v, what I'd expect.
Fuel trim on bank 1 however climbs from 0% to +59% over 15 seconds and O2 #1 is flat lined at 0V.

Going to assume the O2 is bad? Do these trucks need extended driving to set a code or pending code? Only drove about a mile with this horrendous bog, but while the live numbers show a big issue, it didn't set a code.

You could switch the front and rear O2's on the bad side and see what that gets you. The way I understand it, it is the front one that adjusts the fuel trim. If the O2 is bad it will make it show a cat code if you switch them. At least that way you aren't throwing parts at it.

How many miles on the truck? Any other modifications when you bought it, intake or exhaust?

All original, 280,000 km (174000 miles)
Pretty much certain it's the O2, especially since it's so close to the frame and I jacked that side of motor up to re-do exhaust manifold studs..

To answer my own question, a friend is a owner of a decent sized garage and he mentioned Ford's are the worst he's seen for holding off on setting a code or even a pending code.. That's what was throwing me, have no issues with a wrench or electronics but tend to stick to GM. A Chevy that gets no readout from a sensor puts a pending up right away and hard sets it within a couple of mins of run or a couple of restarts..

Will know this weekend, picked up an O2, cheap insurance as with that mileage, old one may throw pissy fits trying to come out.

For anyone pulling this up in search, I hate people that don't finish their thread with the results so...
New O2, did not solve the issue, but did point me in the right direction. With both front O2's now bouncing around like they should, the rear O2 on bank 1 was bouncing around the same as the front, not supposed to do that! Means the cat is bad.

Ordered 2 Flowmaster cats and put them on this weekend. Bank 1 cat had had its front section of ceramic melt into a blob of crap covering the rear section of honey comb. Couldn't even knock it out banging on concrete. Plugged so bad even a shop vac would scream when hooked to it.

New cats in, truck runs like a top, full throttle without skipping a beat..

Thanks for the follow up, and glad you got it!

Yeah, Thanks for letting us know what the problem was!