Climbing in elevation means colder temperatures however there should be enough heat and circulation in the cooling system to prevent freezing.
Semis often use zip up curtains to partially block the radiator and I have been given several reasons for doing so including preventing the radiator from slushing up in extremely low temperatures at high speed.
FYI everybody, the maintenance schedule calls for a coolant drain and fill at 10 years or 150k miles. A 2015 should not need a coolant change. I would at least check the concentration of the coolant and the clarity with an inexpensive coolant tester. If it is not cloudy and good for the lowest temps in your area, you do not need to change it and you certainly do NOT need to flush it.
Flushing could ultimately shorten the life of the cooling system. It is best to drain and refill, but the subsequent changes should be done at 5 years or 100k because you will leave a little old coolant behind each time. The old coolant is less damaging than the minerals you may get into the system from flushing tap water through the system and FAR less damaging than the flush chemicals sold on the market.
Let us know when you replace the radiator cap and it solves the problem for you.
Some of the members here believe that 10 year coolant needs to be replaced every 5 years. 10 year coolant has been around for 15 years, I have not seen any negative affects from it.
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Sorry it took me so long but I did replace the radiator cap and it made no difference.
I did recreate the problem and this time I noticed that when I got into the climb the transmission shifted and raised the engine RPM to about 3800. Thatās when the temp gauge started moving up. So it seems the cooling system is not capable of keeping up at that RPM.
Could you please explain that comment?
Tester
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Iād be surprised if that was the problem, more likely that both things happen because youāre going uphill. Nothing blocking the airflow? Iād replace the thermostat.
Any chance you have a dragging brake?
I just took a look at Rock Autoās picture of the pump and gasket and from what I see it might be worthwhile to remove the pump and inspect the gasket for failure at the seals between the pump impeller and the engine water jacket. If one of those seals is partially blown out circulation at high RPMs could be severely limited.
I agree with @Rod-Knox but will add that you might have severe erosion of the impeller. That would reduce circulation at higher RPM because the impeller would allowed to cavitate. That would have been my second choice but so often the radiator cap fixes the issue. guess not in this case.
Edit: you should not have severe erosion of your impeller and a vehicle this new. That perplexes me.
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I suppose itās possible
But 6yrs seems a little soon, in my opinion
The plastic impellers on modern water pumps donāt erode like the stamped steel impellers of the past.
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I just looked on Rockauto, and the impellers for this engine are indeed plastic and/or composite, depending on how the particular manufacturer chooses to label them
Because they are so rare and often so perplexing impeller issues are often overlooked as are peculiar water pump housing gasket especially those that are bonded composite gasket as seen in the photo at Rock Auto.
This Ford engine has 3 timing chains and a water pump inside the timing cover, driven by the primary chain. I think water pump inspection is going to be the very last thing on my list of things to check! 
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That photo should be posted on the thread re looking for a simple to repair car.
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Could the overheating be caused by the lower radiator hose collapsing on itself?
We finally have an answer to the overheating problemā¦a very small crack in the thermostat housing. It took over 20 minutes of a pressure test on the cooling system for the leak to become apparent. We replaced the housing and thermostat and all is well in the jeep world! Thank you all for your input!!!
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Good to hear! I had a similar oddball overheating problem with my '83 GTI, in Anchorage, in the winter. Turned out the casting for the thermostat was bad: the outlet to the bypass passage was obstructed with casting flashing. Once removed, problem solved.
Post #1. Coolant levels are fine.
Last post. Crack is allowing pressure loss but no coolant loss?
Ok, you say itās fixed.
In talking with the mechanic, I was told that the 3.6 engine is extremely susceptible to air issues in its cooling system. The small crack did allow coolant to leak but it evaporated very quickly. However the amount of coolant lost was so small that no one would notice the level was low in the reservoir. The amount of air that entered the system was enough to cause issues. Anyway, testing the jeep on the highwayā¦ temperature gauge is steady. Side noteā¦the newer wranglers no longer have radiator caps. The system is closed so that the only access is through the reservoir.