Ok, I've held back on suggesting a exhaust back pressure/restriction problem, but maybe now is the time to check for excessive exhaust back pressure, as it can cause the engine & exhaust manifold to run hot after being on the road & the engine to loose power, which you've said happens after running it on the road & it's good and warmed up/hot. So you might use your vacuum gauge to check for excessive back pressure by scrolling down to look here How to Use and Interpret a Vacuum Gauge at scenario 14, to see if it & the temp gun suggests a restriction.
I think maybe the setting is to pre-calibrate the heat gun setting to the ambient air temp, so to get a more accurate reading.

 

I hooked up my vacuum gauge after the truck was good and hot and it was reading around 24 in.Hg. at idle. I rev'd the engine up slowly to 2500 RPM's but the needle did not drop one bit. As a matter of fact it went up a hair. But what's really, really odd, is that when I rev up the engine quickly and let the throttle snap shut the vacuum gauge doesn't drop momentarily. It stays put. And I know for a fact that before I attached the IMRC actuator rod to the IMRC flaps a week ago I tested with the same vacuum gauge and when I rev'd the engine back then, the needle definitely dropped down to around 3-5 in.hg before going back to 24 in.Hg. Strange things are afoot at the Circle-K.

I wonder if the flaps sticking shut would cause the vacuum gauge to not drop on a snap throttle AND I wonder if it could cause the engine to run too hot? Or, just as a coincidence my vacuum gauge stopped working.

EDIT: Tested my vacuum gauge on my 2008 Lexus and the gauge works perfectly. Car held a steady 21 in.Hg. and snapped down to around 8-9 in.Hg on a stab of the throttle. So something is definitely up with my intake. Someone on a youtube video said that if your vacuum gauge reads higher than 19-22 in.Hg. then there's a problem with the engine.

 

Ok, so, apparently connecting the vacuum gauge to the vacuum port on the intake down near the IMRC flaps (where the IMRC solenoid vacuum hose connects to) is not a good place to connect it. I reconnected the gauge to the vacuum port just behind the throttle body and it works perfectly. A solid 21.5 in.Hg of vacuum at hot idle. Snapping the throttle open causes it to drop as expected. And raising the idle speed to 2500 RPM shows no vacuum drop on the gauge. As a matter of fact it went up about .5 in.Hg and so from that I don't think there's an exhaust blockage.

I also used an inspection mirror and my LED light to see that the IMRC actuator is moving the flaps and so now I know that is working properly.

I also used the temp gun on the inlet and outlet of each cat.

- The primary cat registers 450* at the inlet and 380* at the outlet.
- The secondary cat registers 360* at the inlet and 270* at the outlet.
- After the secondary cat at the connection point it registers 200*-230* but
then right at the inlet of the resonator it jumps up to anywhere between
310*-350* which is just weird.

I'm not sure if those cat inlet/outlet temps are good, bad or mediocre. Should the temps be cooler going in and hotter coming out of the cats?

I also found a partial vacuum leak at what I believe is a fuel pressure damper thingamajig on the fuel rail. The vacuum hose was split and so I replaced that.

Sooooo next on the agenda is:

1. Next time I'm off I'll take it for another test drive to see if the replaced vacuum hose made any difference in power.
2. Then I'll drop the exhaust at the connection point just after the catalytic converters to see if that helps with anything.
3. If neither of those help then I'll rig up the fuel pressure gauge and take the truck for a drive to see what the FP is at cruise and WOT. It's about 65psi at cold idle.

 

Ok, more good trouble shooting feedback & I agree it doesn't seem right now from your vacuum & thermal measurements that your have a restricted exhaust problem.
Good find on the suspect split vacuum line. Keep eliminating things in a logical manner & you'll likely come upon the cause for the over temp measurements & power loss. Will be interesting to hear what it is.

 

Knock another test off the list.

Just did a cooling system pressure test. Pumped it up to 15psi. Found that it only dropped 2.5 psi over 10 minutes and I don't think that's bad at all.


Also found yesterday that my MAP sensor is reading 4.05v at KOEO for FORScan. My elevation is approximately 100' above sea level. ECM BARO was reading 29.5 but actual baro at the time was 29.95. I tested the harness side of the MAP sensor and I'm getting a solid 5v across pins 2 & 4 so that's good. I'm going to replace the MAP just because of that 4.05v reading which seems low. I figure it should be around 4.5v or higher with my elevation.

Edit: I'm going to rerun the test with another testing cap and tighten it up even further to see how it does.
Edit-2: Ok, 5 hours later and it had not dropped at all. Still sitting at 15 psi. So the system is leak free. On to draining it and looking for blockages.

 

UPDATE:

Success!! Just got back from a 5 mile test drive and the truck is running 204*-208* ECT and 210*-214* CHT. I'll take it on a 30 mile test drive tomorrow when I make a dump run. I really hope this is the end of the engine running too hot issue so I can move on to fixing other things.

Here's what I did:

A few days ago I replaced the CHT sensor and connector. I noticed in FORScan that before the replacement my cold soak temps were IAT @ 64*, ECT @ 68* and CHT @ 68*. After the CHT replacement the cold soak temps were IAT @ 68*, ECT @ 64* and CHT @ 64*.

Today I drained the cooling system and tested flow through the radiator and block (with thermostat removed). Both had absolutely no blockages. Water was coming out at full volume.

I also replaced the steel tube that connects to the top radiator hose, heater core and degas bottle. Mine was coated inside with rust and looked horrible. I then replaced the electronic thermostat with a 190* non-electric Motorcraft thermostat. But, I left the old one connected to the wiring just so I wouldn't get a CEL. I'll install a resistor soon. Not sure if I should go with a 15 ohm 1 watt resistor or 1500 ohm 1/2 watt resistor. The thermostat heater measures 15.4 ohms.

Any way, and the pièce de résistance, the original owner must have had someone install a Dorman brand "Tee" when the original "Tee" above the starter cracked. Well, that Dorman Tee was soft due to the heat and cheap plastic used during manufacturing and the hose clamp was crushing it reducing the size. I replaced it with a Sharkbite UC417LF brass tee. It was a smidgen smaller than the hose size but the hoses clamped down just fine........so far. Hopefully this Sharkbite tee will work until I can get enough motivation to replace the whole hose assembly. Yuck.

NOTE - just for future searches, pulling out that steel coolant tube was a real pain. But, I finally got smart. I used a ratchet strap hooked on to the heater core connection tube and the radiator support area and just ratcheted it off. It was easy peasy doing it that way.


EDIT: May-11: Did a dump run today (about 25 miles round trip) and ECT stayed around 208*-212* and CHT fluctuated between 238*-245*. I'd call that a success.