Running Hot Only On The Highway

I had a different thread going but under a completely different title that led me down multiple rabbit holes so I'm starting a more specific thread. This problem appears to have just started in the last week or so. Ambient temps in the mid 90's. Truck has 102K miles.
Trucks runs 186-194° in stop/go traffic, city driving, idling, anything under ~60mph. Tranny around 160-180° in the same scenarios.
On the highway doing 65mph+, temps will climb to 206°. The faster I go, the higher it climbs.
(65mph-200°, 70mph-204°, 75mph-206°, 80mph-208° etc.)
@ the same time, the tranny temps climb @ the same pace up to ~204-208°. It doesn't start to do this until after 5+ minutes on the highway, not right away.
I sprayed off the front of the AC condenser & back of the radiator but that did nothing. I unbolted the rad. & condenser & sprayed between them but nothing really seemed to be in there. All levels of fluids are good & no leaks. Rad. cap seems to be working because when engine is cold, the reservoir is @ the bottom line & when warmed up it increases about 2". There's no trash or debris under the truck blocking flow but I do have an aftermarket bumper (Ebay special) w/a winch which seems to block airflow more than stock. I spent a fortune on my LC trying to dial in temp. issues & want to avoid throwing parts @ it.
What's been recommended is:
Rad. cap
t-stat
flush the system
change tranny fluid
add a bigger tranny cooler
I'd prefer if someone had first hand experience w/this tell me what their issue was. So, runs normal temps in ALL situations EXCEPT when running 65+mph on the highway.
What gives?

 

As you go faster you generate more heat. Somethings wrong with the radiator and/or water pump.

Problem started last week? Has it gotten hotter around you last week?

 

My 2008 did the same thing but never over 198. Put a new thermostat in never over 194 now.

 

I hate the mud and only get in it whens there's zero other options. Mine looks nothing like that!

Temps have been mid 90's for over a month. 1st noticed this on Tuesday & I pay pretty close attention to my UltraGauge.

 

No first hand experience and I know you don’t want to throw parts at it but, a new radiator cap and thermostat would be ideal just for the maintenance aspect.

Check the hoses to see if they are weak and collapsing under a load. Hold the truck around 2k rpms in your driveway and see if the hoses start collapsing. *Keep your hands out of the engine bay*

A radiator cap that can’t hold enough pressure could be the issue.

Bad coolant could’ve corroded the radiator. Truck ever been wrecked in the front, radiator replaced? Take a peak inside at the fins.

A failing wp where the impeller isn’t flowing at those rpms would be my last guess.

 

How about post a picture of your front end with the winch and bumper?

 

There's pics in my build thread but I've had it like this since March of 2018. I really doubt that's the issue.

 

I haven't checked the hoses for collapsing but I drove 45mph for 5+ minutes @ the same RPM's as doing 75mph & it stayed w/in normal limits. I'll check them anyways in a bit.

 

Check your lower radiator hose. Does it feel firm(ish) or does it seem excessively soft. If the hose is weak, it is possible for the wp to have enough pull that the hose will collapse under higher rpm.

Edit: I type too slow and get distracted by "aliens" folks on tv and forget to hit reply....

 

Just had my kid hold the throttle at 2K RPM’s for a couple minutes. No collapsing and temp stayed at 186 degrees.

 

Looked at your build thread. Noticed you did not get the Ultragauge until October. Temps might have been high the previous summer you just didn't monitor them then. I suspect that big honking bumper, light bar, and winch is deflecting the air at high speed and you are not getting enough air through the radiators.

 

I didn't realize that! I could've sworn I had it all last summer as well. I guess it's the warmer temps making it get hotter on the highway but why?
It was 97° today. All city driving & anything under 65mph was 190° tops. Got on highway & did 75mph & it almost instantly went to 204°.
(Sorry for the late response. I got a 3 day ban for sticking up for and loving the good ol' U.S.A.)

 

I just ordered new upper & lower rad. hoses, 3 gallons of coolant, t-stat & radiator cap. I'm going to flush the system w/distilled water 2 times before I replace it w/the 50/50 coolant. Coolant looks a more brown than pink and there looks like dirt in the bottom of the overflow reservoir. IDK what that's about. If this doesn't get temps down then IDK what will! Still open for ideas as I'll be all out of ideas if this doesn't work.

 

In my opinion, 204-206 isnt that bad when temps are in the 90s, could be normal. The engine is working pretty hard when you're cruising at 65 plus

But, a coolant flush and stat couldn't hurt

 

Don't mean to sound like I'm throwing rocks at you, but that's about a cooling system that has not been properly maintained. Combine that with the ebay gadgetry and you may have an answer to your situation.

 

I bought it with 77k miles, now has 102k. The coolant in the reservoir is bright pink so it may be an optical illusion looking into the radiator. I dipped a paper towel into the radiator and it turned the towel pink, not brown, so I may just be seeing shit that’s not there.
I’m going to get a syringe and pull some out of the radiator. I’ll post a pic of the color.

 

It will be a lot of work, but might want to temporarily remove that bumper/winch/light bar and then try a 85 mph run.
Also add the Engine Load parameter to your UG to see how much load at various speeds.

 

Wish I had seen this thread sooner. Would have recommended the 160° Motorad T-stat.

Probably not what you also want, but for the sake of being informative, CSF sells an upgraded radiator, with a little larger capacity for the integrated transmission cooler.

 

Took a pic of the actual fluid out of the rad. And showing how it looks brown down in the radiator. The fluid is definitely pink and not brown.

 

What’s your temps? IAT is only about 90 degrees on mine so I’m thinking blockage isn’t an issue.