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.

 

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.

 

First the original posters Jeep is BOILING over, that is not normal and will cause damage if not fixed.
Second there is nothing special about Mopar coolant it does NOT have a higher boiling point, the factory fill is simply either OAT or HOAT depending on the year the Jeep was made, but older Jeeps can use the newer coolant if and only if all the current coolant is completely flushed out. But other than lasting longer there is no advantage to it, it doesn’t cool better or boil at a higher temp.

This is an excellent write up on the differences
https://wranglerjkforum.net/threads/...-engine.26656/

50/50 antifreeze boils at sea level pressure at 220 F, that’s only 8 degrees higher than water, the high boiling point is due to pressure, not coolant
here is an article that explains coolant boiling temp and the relationship of pressure and boiling temp.
https://flex-a-lite.com/blog/how-rad...20the%20system.

Finally the temp you see is NOT the highest temp there is, it’s the temp where it’s measured, the heads are significantly hotter as 50/50 coolant boils at 250F or so at 16 lbs pressure. I think the newer Jeeps may have a higher pressure cap, I’ve read that the Cherokee can be as high as 22 lbs?

Do not increase the coolant mix to more than 50/50 because coolant AKA antifreeze does NOT cool as well as water, yes you need coolant for many reasons corrosion protection being among the highest so do use antifreeze just don’t exceed 50/50 and for those of us that never see cold temps we may should consider a leaner mix.
https://www.motortrend.com/how-to/gl...water-coolant/

 

To continue I’m going to make a few statements that some may disagree with and some may see the logic, almost every “fact” posted on any forum that’s not backed up with substantiating data is usually an opinion. Mine included
I did the redneck test and put my thermostat in water and set it on the stove to heat, I used my NIST calibrated Fluke multimeter to read the water temp, around 206 I noticed it cracking open, the water boiled at 212, the thermostat did not fully open at that point but as I could get the temp no higher I quit. I guess I could put it in my oil bath fryer and repeat the test as oil can get stupid hot, that should tell me the full open temp, but if the Wife sees it, it’s not going to make her happy.
Based on that it appears we likely have a 205 rated thermostat. If so it’s 10 degrees hotter than average has been for decades so logically we should see on average 10 degrees hotter than average, but we see higher than that. There are two ratings for thermostat, the cracking or opening temp is the lower number and the fully open number is the higher. Usually there is about 20 degrees difference. The higher number is only very rarely given, as I can’t even find the lower number for ours I have little hope at finding the higher.

In my opinion a well designed cooling system will always operate as a min at least at the lower number, and never exceed the higher number, if you are exceeding the higher number then you are generating more heat than it can dissipate without getting higher than designed. Now as the differences in the water temp and the airflow through the radiator increase the amount of heat the radiator will shed increases so once the T stat is fully open that is not the max cooling possible but beyond that temps tend to climb rapidly as other than the temp difference there is no way to increase cooling.

There is nothing magic about our Pentastar engines, there is no special metals, it’s not “designed” to operate at higher temps, it simply is if pushed hard and or at high air temps, slogging through sand etc. I believe it does in fact dump much more heat into the cooling system than normal for this sized engine because it has no exhaust manifolds, they are cast into the heads and as the heads are cooled by coolant that extra heat adds to the cooling system load.
High head temps among other things will cause lubrication problems in the head, will raise oil temps as oil is circulated in the heads and don’t forget if your coolant temp is 240 then your cooling your oil with 240 degree coolant, how high does the oil temp get then? Almost certainly over 250 and while good oil will tolerate that it’s awfully thin at those temps and remember most of an engine is actually oil cooled, cams, cam followers, the crankshaft and it’s bearings etc.

There're some advantages to cast in exhaust manifolds and the Pentastar isn’t the only one, many engine designs have gone that way, most of the advantages are emissions related. However it seems to me that Jeep didn’t increase the cooling capacity when they increased the heat load and in some other vehicles they are jacking up the coolant pressures to enable even higher temps. That is if some Cherokees actually are running 20 lb or higher caps.
‘In my opinion we would be better served as in our engines would last longer etc if they ran within the thermostat operating temps. As weather cools even those of us in hotter areas will and those in cooler climes when not running hard always do. I’m going to investigate increasing the airflow through the radiator first, then investigate a higher capacity radiator, I think a higher cooling capacity radiator would be the best fix myself. Many if not most “all aluminum racing radiators” don’t actually cool any better, they just look cool. But I think, maybe there may be a larger factory radiator that may fit, but I don’t know that. Wasn’t there some kind of higher HP JK like there is the Hemi JL? If so maybe it’s radiator will fit? If it’s a higher capacity radiator.

Fact is as designed if driven as designed with the small factory sized tires just as road vehicles back and forth to work etc there is no issue. However I think as start modding them for off road and the low speeds and higher engine loads associated with it we have cooling “issues”. Same for people that tow heavy of course especially in hotter air temps. Maybe we should be modding our cooling system as we mod our jeeps and take them off road, at least for those that do so in higher ambient temps and although it doesn’t seem logical because it’s usually cooler but if your at higher altitude due to decreased air density your cooling capacity diminishes with air density.

Installing a lower rated thermostat will only lower the temp you operate at in cold weather etc. it will not do anything to bring down those 230+ temps some of us see. In short it could do more harm than good because too cool isn’t good either,