Hi all! Another update:

Feeling more than a bit out of my depth, I took it to my regional LR specialist to get an opinion. They were happy to review the steps I had taken so far, and ran a few diagnostics including testing my fuel pump (it's OK), looking for vacuum leaks (there are none) and they managed to replicate the issue. They noticed something that didn't occur to me to even check: while running rough, two of the O2 sensor voltages were "stuck". This prompted the tech to check for exhaust leaks. Turns out I have a cracked weld between the 2 O2 sensors on the driver's side, as well as more than 1 failed manifold gasket. They recommended a new exhaust system with gaskets and O2 sensors.

I said thanks and paid for the diagnostic, excited to have a solid lead and something I can actually try and fix. Had an uneventful ~30 mile drive home and just as I finished parking, ding! Pending trouble code, driver's side O2 sensor voltage out of range.

Still seems weird to me that an O2 sensor missing due to a leak would only be triggered by running hot and not just reading wild all the time, but OK. I also think having exhaust flowing right from the block to the engine bay is probably stirring up gremlins and explains why my engine bay is so hot when driving slow.

I'm setting up to order new manifold gaskets, all the studs & hardware, y-pipe with new cats and O2 sensors. A friend of mine is being so generous as to lend me his garage so I don't have to go far to do the work or try it on the side of the road. If all goes well, I'll have some story to tell in a few weeks.

Something else that might be related or a separate issue that I noticed on the drive there and back: since replacing the MAF, if the intake temp is over 95-100 degrees, the MAF voltage jumps around from the true reading, something like 0.3 to 0.5 g/s on the UG to 0.00 up to 2.0+ and while the value jumps up or down the engine surges, RPM's are steady but you can feel the truck trying to react to erratic MAF data. If you keep driving and cool the engine bay down below 95 degrees it will stop 100% and run normally, if you stop and let the intake temp creep up the MAF goes weird again. Any ideas what could be causing that?

Thanks again to everyone for offering their experience and advise, I feel like I'm back on track to tackle this one.

 

Another update:

After the diagnosis from the LR shop I got to work repairing the exhaust… pulled the manifolds and y-pipe, replaced all of the manifold gaskets & hardware, some of the old gaskets were rotten and leaking obviously. Most of the manifold bolts were only finger-tight, the washers and locking tabs had mostly rotted away.
I also installed a new y-pipe with cats and 4 new O2 sensors. It was very easy after taking the sway bar off, only one broken stud in the manifold, which I was able to drill out. Got everything put back together and the total job took probably 8-10hrs over the course of a few days. No power-tools, just a pair of 9-inch ramps, it was definitely a bit of wrestling to get the old y-pipe out but not too tricky.

The truck sounds a lot better, some under the hood noise, worrisome engine ticking etc. turned out to be all exhaust noise and the whole thing is much quieter, very nice!

Took it for a short test ride and everything seemed great, drove as well as it ever had around town, got everything nice & toasty and no gremlins. After work I took it on a longer drive, got it up to the magic temp where the intake air sensor was reporting around 125 degrees, and it was still great, O2 sensor readings were much more dynamic than with the old ones (2 of the old downstream sensors were stuck at “1.015”), MAF sensor reading was solid and consistent with skinny-pedal action. Got brave and took it to the interstate and right on the on-ramp the gremlins appeared. Giving it more gas to get up to 50+ mph resulted in sputtering, backfiring and a loss of power.



HOWEVER this time it was short-lived. Only lasted for a few moments and it went away when I tried WOT, and eventually got my normal power and revs back. MAF sensor readings were all over the place but everything else seemed fine. I chickened-out and turned-around and the ride home was totally normal.

Kind of scratching my head now, wondering if somehow MOST of my issue is resolved but there’s something else missing obviously. Maybe my O2’s AND MAF were bad and causing similar symptoms but now there’s only half of the problem present and the symptoms are milder.

I’ve kept reading and researching and cannot imagine anything that would cause fluctuations in the MAF sensor besides a faulty sensor (the g/s reading is just not realistic). But it’s so weird that it only acts-up in a specific temp range. Is my ECU overheating? Should I just buy another MAF? I might try unplugging it for a while to see how it runs on the default values. Kinda bummed that all that work and new parts didn’t crack the case, but that stuff needed to be done. I also did my PCV tune-up and fought back some rust while I was at it.

 

It’s been about 5 months since the last update and I’m happy to report I will consider this case closed and the truck has been running mint since finishing the repair.

To make a long long story short, the MAF / Air Meter was the last part of the puzzle. I decided to change it since it was the last variable, and something the LR shop told me they weren’t even going to look at (because it was aftermarket). I also had reported earlier that while the engine was getting hot and the gremlin was waking up, I had gotten erratic MAF readings from the ECU. I found a good deal at a local junkyard on a low mileage part, took some refurbing but noticed the sensor values were much more responsive, no longer erratic when hot, and the rough running issue, which I’ve now classified as a bad over-fueling issue, has not returned.

Big thanks again to everyone who offered advice, hopefully this thread will be helpful to someone in the future diagnosing a similar issue!

 

Thanks for the update. Happy to see you got that Gremlin