You could go to a junkyard and pull off a module and save $190 something just to throw it out there. I have a spare in my trunk because I did have an intermittent overheating situation for a minute. Not 100% like yours but mine was with the AC on, going up a 7% grade for 5 miles when it was 118 degrees outside. It happened twice. I replaced the T-Stat and it never happened again but who knows.

My second opinion would be like the post above me, look into a wiring issue.

Eh, I wish we had a JK with some dead Panthers to gut.
Not in Poland in Central Europe.

Anyway, I considered a fan controller issue, but I don't think that's it.

If you're speaking about your '10 P7B, you got a rad fan which has stepless speed control, and that's what FoMoCo started putting in these cars somewhere around '04 or '05.
On my '02, I got a two-speed fan which is commanded by PCM via two relays, one for low speed, the other for high speed. The low speed relay, if commanded to drive the fan, does so supplying 12 V through the fan's controller which is basically a big ass resistor. This resistor steps down voltage, so the fan runs slower that it runs when it is driven through the hi speed relay, downstream of which there's no resistor. This prevents operation at high speed, which would otherwise cause the car to heat up too long (and the cabin heater, too) and wear down the rad fan from running at max speed all the time. Simple.

I just found it all in the FSM I unearthed. So it's either PCM's command circuit on the fritz, or the hi speed relay, or something else electrical in between. The system is simple. I know fo' shizzle that my PCM harness plug is intact, no evidence of anything which could cause the hi fan speed signal to be interrupted on the component side, so I can only trace the harness (which would be a great idea after what I found in the engine bay harnesses while troubleshooting the RAS power supply circuit - WORN DOWN INSULATION AND MMMM YUMMY GREEN CRUSTIES!).

You might want to pay attention to the relay box contacts. Some of us have found problems there...corrosion, spread contacts and the like.

Have you figured out if it's definitely related to running the heater/ac? The AC should also have a wiring connection to the radiator fan, as those diagrams I linked show.

<b>2010mgmUltimate</b> I somehow couldn't open the PDF files from you. No biggie, I made my FSM run on a virtual machine with Windows XP.

I am still digging in the schematics. I've ascertained by testing that this only happens when the EATC is commanded to blow air through the floor vents. It is when the radiator fan somehow stops running. I'm kinda lost as to why, but then again, I didn't know jack crap about wiring when I began my epic quest to make the ride air system work again.

What I know is that with any other setting the car won't overheat. I ran her for 40 minutes in heavy traffic, then I took her for a wild spin down the city's bypass at 70-80 mph for 30 minutes. Nothing. She ran like a champ.

What bugs me is something completely unrelated to how PCM controls the fan, and it's the temp sender which lives on the coolant x-pipe of the intake manifold. I got the digi dash and somehow, due to a weird quirk of hysteresis, one bar above the mid scale of the temp gauge comes on, then it goes off. It's somehow consistent with something going on in the cooling system, either the t-stat or the fan switching between low and high, don't know yet, but the temp gauge follows the temperature fluctuations picked up by the cyl head temp sensor and the coolant temp sender I monitor with live PIDs over OBD II.

Neither could I...

The PCM defaults to fan high speed whenever the AC compressor is requested/needed. This happens in all but two modes: Floor or Off. The rest of the time, I believe the fan speed is controlled by the engine temp.

I was about to upload them to GD instead, but I just read your statement that "he PCM defaults to fan high speed whenever the AC compressor is requested/needed. This happens in all but two modes: Floor or Off." In that case, then I'm probably wrong, since he said it overheated in floor mode.

If the rad fan is commanded to run at high speed with the floor vents selected manually, there's still something funky going on with the command circuit between the PCM and the rad fan; see, with EATC in automatic operation, the fan switches between low and high without issues.
It's still weird, because the fan speed is governed by switching between the fan power relay which drives the fan via a resistor to run at low speed and another fan power relay which drives the fan directly (as the resistor on the fan is the only thing which makes it run at low speed). So there must be a reason why, with the floor vents on, the rad fan suddenly stops working altogether, causing the engine to start overheating slowly (in idle).

That's exactly what led me to think it was a super weird wiring issue with the circuit that controls the rad fan speed when the AC is running.

Have we eliminated the thermostat by replacing with a new one?

Nope, but the thermostat dumps to the radiator every time, I'm pretty sure of that. The issue is with the rad fan.

It might be a super weird electrical goblin. I've had zero time to do this (work, argh!), but I need to retrace the EATC wiring to the harnesses which cross the engine bay bulkhead and possibly, the harnesses in which the rad fan command and power wiring lives.

There's this one glitch related to EATC which keeps bugging me; when there's high humidity or the underside gets wet (puddles!), EATC has a funny habit of switching the temp display between degs Fahrenheit and degs Celsius - which you can do manually by pressing and holding both outermost buttons on the EATC panel. When I run through a deep puddle, three things happen: the dash clock, EATC, and radio head unit backlights sorta dim out for a second or two. Note that this happens when the right front wheel hits deep water.

There are some things interrelated in the wiring on these cars; the 'puddle problem' was also evident by killing the head unit power... until I repaired the harness which lives within the right front wheel arch cowl - green crusties on a power wire just ahead of the firewall grommet, and that power wire was why my ride air system wouldn't tick (it has several splices and connectors in the cabin, for various systems). With that repaired, RAS started working and the head unit stopped being clapped out by water splashing from under the front right wheel.

I'll repeat myself, but I've seen other harness issues 'by courtesy' of FoMoCo quality control; I actually cleaned out two more harnesses in the area of the cabin fan and the A/C accumulator, wrapped them in thick tape and finished off with a wrap of textile tape; then came the zipties to hold the harnesses away from any component against which they had been rubbing.

Back to the FSM wiring diagrams, methinks.

Hi. I had a problem with the radiator fan not working on my 2003 CVPI that drove me nuts. I finally took it to a reliable shop that found a corroded/ broken wire in the bundle of wires /harness directly under the battery tray. They repaired that and it was back to normal.

They also found a bad connection on the underside/ bottom of the fuse/relay box aka battery junction box for that circuit.

Good luck!!!

It also might be a super simple worn thermostat. But, if you WANT to make this into a complex electrical discussion for the sake of posting words rather than following a basic trouble tree, then good luck on the fix!

I do not ignore a potential t-stat failure, although it would be a super weird causality between it failing to open every time the floor vents are on with the cab heater.
To clarify, the t-stat was replaced some 4-5,000 mi ago with the intake manifold. It still might cause problems due to a manufacturing defect, who knows? I still really like to know why the fan fails to run with the floor vents on and, possibly, the t-stat stuck closed as you suggest.

Sure, I can pull the t-stat and test it in boiling water, tho I planned to change the coolant some time next month (I'd have to do both at the same time, for convenience's sake).

Oh, I only suggested it as a good starting place to look ... before you found the fan was not working properly. Without the fan at idle sitting stationary, little or near no air goes through the radiator like when motoring along at 40 on the Parkway.

Max air will turn it on usually, even if temp control is set to hot ... also good for defrosting windows.