Might be helpful to put the upstream O2 sensor on the Dashboard, so we can see its switching range & graph it too, so we can see the response curve & speed.
It's a maintenance replacement item & with at least 168K miles before the odometer stopped working, if not replaced, it'll have lived about 2 lifetimes by now, so may be like me, old & slow to switch!!!??? lol

 

I'll add that to the dash and check the model number as well.

 

I forgot to reply to the 3, (beginning, middle & end) screen shots in post 42 when it was misfiring. Fuel trim numbers there look ok, nothing wild going on, so right now stay more focused on a spark issue, as you haven't raised the coil packs to spritz them down underneath after dark on the wet down test. You could raise them cold, sprits them down, then drive it to warm things up, then do a hot wet down underneath & see how it goes.

Intermittent problems are the most difficult to diagnose & the ones Dealer Techs dislike the most, as we have to be monitoring things when they happen, to know, or get a good idea of who goes to the top of the suspect list.

Right now it looks like fuel trim is probably ok & a miss-fire under load at higher rpm, usually puts spark at, or near the top of the suspect list, but your first wet-down test didn't show anything suspect going on Externally with the coil packs, plug wires on that wet-down test, so if all goes ok Externally on the Raised coil packs wet-down test, your sorta left looking at a internal problem with one or more of the electricals associated with the cylinder/s giving the misfire code. Things like chipped, cracked or dirty Internal or External spark plug insulator, or something like dirty, or carbon track, or faulty plug boot., or some kind of inside boot insulation problem, faulty / loose plug wire end connections, or faulty insulation in one or more of those, causing a under load misfire.

So sometimes we have to swap parts to see if the problem follows the part, rather than tossing parts at the problem on a hunch. So we can see why Techs trying to make a living dislike working intermittent problems!!!

So don't get discouraged if you don't get joy right away, All trouble shooting results are trying to tell us something, we just have to interpret them correctly to come by Lasting joy. lol

 

I just noticed your edit today. I think I'm going to try replacing the plugs; RockAuto has a good price on the Autolight XP104. Do you think Autolites will be fine, so long as they're iridium and platinum, or do I need Motorcraft?

 

A full spray test is on my to-do list.
I added the O2 sensors to the FORScan, and have had one drive so far with them on. They indicate "No Fault". If it misfires again, I'll post screenshots.

 

autolites will work fine. It is some what of a bas***d to replace them on driver side. You may want to have new wires on hand if old ones are suspect. Again autolite or Motorcraft.

 

The specified heat range Autolite fine wire XP plug will probably be ok, they have been fine in the Wifes 2.0L Neon waste spark ignition for years now.
The Specified Motorcraft fine wire plugs have been fine in the 99 Ranger 4.0L for years.
Its thought that Autolite makes Motorcraft plugs to Fords specification. That seems plausible as Ford began to offer the Motorcraft fine wire design shortly after the Autolite fine wire XP design plugs hit the market.
As noted in the spark plug tutorial, use a Wire gauge to check / adjust gap & Dont pull / force it through too tight a gap, so as not to damage the fine wire center electrode, or ground side wire platinum pad.
I choose a wire gauge that meets the Min spark gap, as gap will stay in spec longer over time (as the spark gap is always eroding to a wider figure), then check my setting using a go / no go set-up by selecting wire gauges that are .001 larger & smaller than the gap I want. The smaller gauge goes through easy / no drag, the larger gauge won't go, so I know the gap is where I want it.
Gapping to the small side of spec will yield a hotter spark kernel that can help get us going on those cold morning starts, when lower battery voltage causes the ignition system to work its hardest to make a good hot spark kernal to ignite that cold harder to light off air / fuel mixture to get us going!!!
Will be interesting to hear if using the specified design, heat range & spark gap plugs will make a noticeable difference in how it starts & runs.

 

Alright, I'll order up a set of Autolite plugs. I'll let you all know how they affect performance.

I drove the truck a bit more today, and on one trip, I had one misfire on cylinder 3, and one misfire on cylinder 4 on the way back. The distributed nature of the misfires, and fact that they aren't always prolonged is making me think it's the plugs getting fouled occasionally. I'm still going to do that coil spray test though. Got a finish up another project so I can get my truck in the garage to test it.

 

Looks like the XP104 is the OEM Motorcraft replacement, Parts Finder - Part Detail (autolite.com)

 

What on earth?!?
I got the new autolite plugs today and am working on installing them. Look what was in the driver’s side of cylinder 2! I mean, how am I not getting a Cyl 2 misfire? Anyway, it’s definitely getting replaced. All the other plugs are NGK, this one is Champion, so apparently the previous owner skipped this one when he was replacing the plugs. I think I’m definitely going to notice some better performance with this gone.
I’ll keep you all updated.

 

Dang thats a nasty looking plug & the gap looks to be Way out of spec Wide & that'll stress the coil pack & wires carrying the extra voltage to jump that gap.
Use a shop vac & a round wire brush to suck up & clean the spark plug threads so the new plugs are making proper electrical & thermal connection to the head.

 

Yeah, I didn't measure the gap, but it's definitely way too big. Is the ground electrode eaten away like that from the waste spark?

Last night I sprayed down all sides of the coil packs, and saw no evidences of arcing, sparking, or barking. I took the truck out for a short test drive with the new plugs, and it didn't misfire, and seem to be running solidly. I noticed I'm not hearing the crackling sound I used to hear when the engine was under load. If it was unignited gas being burned in the catalytic convertor, that seems good evidence that the new plugs are giving a more complete burn.

That's a good idea to clean out the threads. I'll do that.

I'm going to take it on a longer test drive today with the FORScan attached to make sure I've got this fixed. My version is FORScan Lite v1.5.14 for Ipad, and my adapter is ELM327 v1.5.

 

Not sure which model Champion plug that is, nor how they designed it to look when new but I think I see something on the end of the side wire that looks like the remainder of a pad of something, maybe its just deposits, or got mucked up with debris during removal???
If that plug was from a cylinder that got the Reverse polarity spark & only has a standard / non enhanced Nickel side wire, without at least a platinum pad enhancement, would suffer a Lot more erosion in a waste spark ignition, that works its components Twice as often, as it makes spark on Every rotation of the crankshaft!!! Same goes for the Center electrode, without some kind of enhancement, they just won't last long in a waste spark ignition.

When you did the latest wet-down test, were the coil-packs Raised, so you could spritz underneath too??? Remember, on a cold start the 4banger is only using 1 coil-pack, until the O2 sensor warms up & comes on line / changes its Closed /Open loop mode, so be mindful of that when you perform the wet-down test. You can Safely verify when Both coil-packs are online with your inductive timing light.

 

I did lift the coil packs up. I did not, however, verify that both were engaged. I’ll make sure both are running when I do it again.

 

Good feedback on the ELM model & version of FORScan being used. On the data drop-out noted in post 34, I forgot to ask, is this a Bluetooth, or Wi-Fi broadcasting ELM??? If Bluetooth, it might get overloaded & suffer data-dropout if trying to broadcast too much fast-changing data, or if the signal becomes too weak from distance, or a heavy load of fast changing data + distance or something blocking the signal, or something like a cell phone, or tower near-by, or power lines, or even a electrical storm near-by, can cause data drop-out if its electrical "noise" is on the same frequency. So would would be insightful to know how the ELM is broadcasting its data.
My ELM is the same model & is Wi-Fi & so far it'll broadcast 60 ft with a heavy data load without drop-out.