Right off the bat here, I am sorry. I bet this subject is covered here extensively. I am try to hurry and diagnose this issue and get it fixed this evening. Or at least a game plan set.

2017 Rubicon, wife's DD. We purchased it new and it has just short of 68k miles.

Earlier this year it through a code relating to thermostat or sensor or something I don't recall. It did not over-heat and seemed fine but I changed thermostat anyway.

Fast forward about 7-8 months and it started to get hot. Had the wife turn on the heat and keep an ey on the temp to get it home from work. I boiled the thermostat and it did not open. I grabbed a new one and boiled it too. It did not open either, but chalked it up to being at 4600 ft elevation and put it in anyway. I also replaced the radiator cap.

I was fine for several weeks and just today she was on her way home and noticed that a temp was not registering on the guage. I told her to be sure it was set to hot and wait for it to possibly get warm or hot really fast, which it did while we were talking. She said it was still not registering a temp but the fan started blaring and the air being blown in the cab was hot. I told her to watch it and get home since she was not far. At his point it started to spew steam and she pulled into a parking lot and left it cause she had an appointment to get to and had to hurry and get a ride.

I was an hour away and was going to limp it home or tow it. I find out the step son goes and fires it up and drives it home (maybe 1.5 hours or so had passed), he said it was blowing cold (ambient I am assuming) air was blowing into the cab and then it got hot as he pulled in the drive way.

WTH? 2 thermostats this year and it still boils over. Everything is stock has original fluid (I know I should have changed it by now) and nothing added to grill openings for looks. Right now I am going to go out and house the crap out of the radiator to see if anything is blocking the fins. I would be surprised to find much in the fins. Since getting my Can-Am we do not use this offload anymore. In fact I was starting to get it ready to sell soon.

Any helpful suggestions or ideas would be great.

Thanks

 

I had a similar issue and took me quite awhile to figure it out. Mine would rise above 210 degrees and stop about 230. After changing hoses, adding new antifreeze, thermostat and housing, waterpump and then changing thermostat again to a race style early opening unit. All of that - no real change. Then I noticed the fan was operating louder than normal. It came on so gradual ,took me awhile to notice loud it actually was. So I changed the fan ,bought a factory mopar unit. Very expensive. But that did the trick. Everything finally back to normal (for a Jeep). I assume at speed the fan was slipping causing the hi temp.s and loud noise. Might check your fan.

 

I went and dinked with it after posting this. I noticed it blew the hose off the thermostat and drained from there. I hooked it back up, filled everything back up so I could run it and watch things.

It went to 212 then dipped to 203 then back up past 212 to about 228 then a few minutes it would go back to 203 and would keep cycling like this. I am going to make sure it is still topped off tomorrow and of course if not then top it off and see if it cycles the same way.

What should the range be? I miss the old days when it was simple guage with a dial lol, now we focus on numbers. I used to look and see the needle point straight up-ish and give a thumbs up.

Side story...
Like the damn tire pressure guage giving a number for each corner. My wife would complain that one is at 40psi while the other is at 39psi, you wouldn't believe how many arguments this has started. I pull out the good old pocket guage and say do you know how many years these were the tool to do this? LMAO I aired my Can-Am down and told her that one was at 11psi, two at 10psi and the other 2 at 12psi. She kept asking if I was going to make them all the same. NOPE.

Sorry, went off subject there

 

I just bought mine a month or two ago, eventhough it only had 26,000 miles on it as it was eight years old I decided to drain and flush and put in new Mopar coolant and T stat with a metal neck.
Now I’ve been a mechanic for a long time so I know how to bleed a system and I bled this one very well as in let a significant amount of fluid flow after there was no more air, a lot actually as I was using that to get as much water out as possible so I just let it flow until I got color indicating coolant, so it was very well bled.
Started the Jeep and it started to get real hot pretty quick as in I think 230 or so, so I shut it down, did not open the cap of course but did open the bleed valve, got some air but not a lot, closed the valve when coolant began to come out, being hot it was under pressure of course when my first bleed was not and I think that may have been the difference, restarted and it went from 230 to 150 or so indicating that there had been little or no flow, since then normal temps, hotter than I like and I’ll look into installing a “push fan” to see if that helps. I think our motors run hotter than most, I think it’s because we don’t have exhaust manifolds and our radiator or possibly fan isn’t big enough to keep the temps where I like them

Anyway if your getting weird temp cycles it’s very likely air in the system, if the pump gets a slug of air it cavitates of course and water flow stops, eventually the air moves through, the pump gets some water and primes and then temp drops.

In the old days the air would just go to the radiator because air rises and this was the water flow direction, bottom up, and this is why after a drain and flush we would have to re fill the radiator when the air worked itself out after running for awhile.

Long time ago most cars had no water pump, water circulated from the bottom to the top by thermal siphon and air rises of course so the self bled, later pumps were added and cars until the early 90’s or so had pump boosted thermal siphon cooling. NASCAR i believed figured out that the heads were the hottest part of the engine and the bottom up flow of water did not cool the heads well as the water was already heated by the time it got to the heads, so they reversed the flow to top down AKA as “Reverse flow” cooling system. Factories I assume saw the logic and followed suit, my first reverse flow cooling car was a 93 Z28 Camaro.
So being reverse flow and air of course rises to the top it’s now tough to get the air out of cooling systems, I assume but do not know that our Pentastar being a modern design is reverse flow.

If your having trouble bleeding the system it might be worth taking it to a shop that can pull a vacuum on the cooling system to get the air out, as you know bad things happen from overheating, blown head gaskets and warped heads being probably the most common, so I wouldn’t drive it until it’s fixed myself.

Of course all of this is my opinion and while I’ve been a mechanic for a long time I am NOT any kind of Jeep expert so take it with a grain of salt.

Oh, another opinion, don’t run more than 50% coolant because the more coolant you run the poorer it cools, coolant does lots of good things but it doesn’t cool as well as water.

 

The Mopar premixed antifreeze in a 3.6l JKU boils at a higher temp. than older generations of antifreeze which allows the engine to run hotter than older engines. I don't believe it is uncommon to see and maintain 230degrees when pushing the engine hard. Mine has always backed down to the needle straight up and down I get off the throttle. My experience shows any temp. indications between 210 and 230 degrees is fine as long the system is tight. Once all air is purged, if still has temp. surges, sounds like another bad thermostat.I still have the original factory radiator in my 2013 JKU and am surprized I do not have any issues with it - knock on wood -

 

230 is completely normal. I’ll drive up a long hill at altitude (mountains in AZ) in 100 plus degree heat and let my engine sing at 4500-5000 rpm. My coolant temp will climb to about 240 degrees before I start to back off the gas a little. It will cool down a little and eventually return to 220-230. I’ve done this for years and never had an issue. I think the temps you’re seeing are completely fine.