Digitek14

It's about time I actually posted on this forum. My 2012 JKUR 3.6L Automatic has been overheating recently. It only happens when climbing hills, running at high RPM's, or continuous driving on hot days. The temp gauge slowly climbs up to about the 3/4 mark then drops back down when I let off the throttle. It remains cool while idling and driving lightly.

I recently flushed the coolant system and refilled with fresh coolant. I pulled all the hoses and manually flushed the radiator, heater core, and engine block with distilled water. The flow appeared good in all places and the old coolant was relatively clean without any sludge (used a cleaner as well). I then filled it up with distilled water and ran the motor up to operating temp, drained it, flushed through the hoses again with distilled water, refilled with 50/50 mix, and then bled all the air from the system. I did use the Prestone universal coolant (to get away from HOAT/OAT) and have heard of people running this in the Pentastar motors without any issues. The bizarre thing is there seems to be a possible correlation between how much Prestone I've added over time and how bad the overheating has gotten, it seems to have even gotten worse after the flush. Now this may be coinciding with another issue, as I have a hard time believing a fresh batch of coolant would perform this way, especially with the good reports of others using the Prestone.

I just replaced the thermostat during the flush, the radiator fan seems to be functioning properly, the serpentine belt is tight, the water pump pulley spins smoothly by hand and feels fine (no leaks either), the temp sensor appears to be working correctly, no hoses are kinked or clogged, and there are no apparent leaks in the system.

Any ideas what the issue could be here? Could it actually be the Prestone coolant? A dying water pump? A bad radiator (fins seem fine)? A bad radiator cap? Or something else?
I am leaning towards the pump but could it lose performance without dying like this, and is there any way I can test it if so?

Thanks for any ideas!
Ian

Jay2013jk

Welcome to JKF.. Question is it boiling over? Automotive engineers do not consider an engine overheating until the coolant Boils.. It is interesting that you see 3/4 on the gauge and then when back to idle it returns to normal.. When I replaced the thermostat on the 2013 I had my Jscan hooked up when "Burping" on an incline (front up) and got the temps up to 205. I did replace the rad cap with a MOPAR correct for the 3.6L. then all the air finally exited the system; that was over 1.5 yrs ago... Good members here and you should get more feedback and suggestions.

resharp001

Well, "recently" would include over the summertime and I know those temps in the Springs there can still get pretty warm. What you explain is fairly common IMO. I would also say you need to monitor actual temps from the computer and not just the cluster gauge. I want to say that 3/4 mark on my 3.6L is ~240° +/-.

Presume you've made no bumper, gear, or tire size changes recently or added any bumper accessories like a winch or light bars.

There should not be any issue with you switching to a different coolant as long as you flushed the heck out of the system to get all remnants of the old coolant out. It sounds to me like you did a pretty thorough job doing so.

My personal opinion would be to continue to monitor things for the moment. I don't think it's the WP myself. I'd be more concerned if I started seeing temps rising in more moderate driving which you say is currently no issue. I'd also pull that cap a couple mornings in a row when the system is cold and just confirm that coolant is right up to the top of the neck. These systems are always stubborn to bleed and it often takes me a couple times of topping off before it get it completely bled or air.

Not sure how many miles you have on your '12. I replaced the WP on my '13 with maybe 95k miles on it and only cuz I had the radiator out of the way and access was much easier. The impeller and pulley seemed fine though there was a lot of pitting on the gasket which I found odd. Bit silly we have to remove the alternator to swap the serp belt on this 3.6L.

XJRefugee

Digitek, do you know the boiling temp. for the Prestone Universal coolant? I ask because the newer HOAT type coolant boils at a higher temp. than the older style coolants. As Jay2013 states overheating does not take place until the coolant boils (which is where engine damage will start to occur) The HOAT boils at a higher temp. protecting your engine if the temp. climbs to to 3/4 mark and then returns to middle of the gauge. Resharp mentions during summertime he sees higher temps, my experience is same as his (I am in the Arizona desert) and I believe it is due to the HOAT coolant not boiling at a lower temp like the older types of coolant. I would run the HOAT coolant only.

Digitek14

Jay2013jk,

Thanks for the "warm" welcome! I have not heard it boil yet but it only happens under the heavy motor loads and I've been afraid to pull over and lose airflow while at these high temps. Based on a comment XJRefugee made below though, I looked up the boiling temp for the Prestone and found it was only 265°F and I think temps are approaching that when it heats up. The 3/4 mark is about 250°F on our 3.6L right? Apparently the HOAT does not boil until 400°F. If that's the case I think that sounds like the problem based on the correlation between how much Prestone was added over time with the overheating. It never used to exceed to halfway mark. Think I'm going to try replacing the radiator cap, then switching to HOAT, and reporting back.

Digitek14

Resharp001,

Good point, I'll try checking the temps from my scan tool and see if they are matching up to the gauge reading. It seems to be functioning properly but doesn't hurt to check.

No changes to wheels, drivetrain ratios, or extra weight added recently. I did replace the belt tensioner, idlers, power steering pump, and harmonic balancer recently due to it shredding belts. I checked the water pump at that time and it feels fine, and I can't imagine the impeller losing performance without complete failure. So like you I don't think that's it.

Definitely flushed the crap out of it. I keep checking for air and bleeding by running the motor and defroster with the cap off and squeezing the radiator inlet/outlet hoses and can't seem to get any more out. That's super helpful to know your water pump seemed fine at 95k though, that's what I really wanted to hear. Mine is at 114k so it too is probably fine, and no leaking from the gasket. I am starting to think it's the coolant based on its significantly lower boiling temp (265°F vs 400°F see other replies for more details). Thanks for the feedback!!

Digitek14

XJ Refugee,

I think you may have just helped solve it. Apparently the boiling point for the Prestone is rated at ~265°F whereas the HOAT is rated at ~400°F!!! Though there is not a direct correlation between boiling point and heat capacity of liquids due to other factors at play (such as density, vapor pressures, and intermolecular forces), there is a general trend between the two. I'm thinking the HOAT will absorb/transfer heat easier in addition to boiling hotter.

I thought this sounded insane that the fresh Prestone would work this poorly compared to HOAT (especially with the supposed good reports from others), but now that I'm looking at the specs it seems clear. This is the only recent change that seems to coincide with the heating as well.

I'm going to swap the radiator cap first, see if that does anything, then flush and replace with HOAT. I'll keep you guys posted!

XJRefugee

In other daze I had a 99' XJ Cherokee with the good old fashioned 4.0 liter 6 banger with 275,000 miles on the block. I changed out the normal coolant and water with EVANS waterless coolant and drove that way thru a hot Arizona summer. While it did show higher temps when driving normal, there was no affect on performance. Climbing out of the valley up a long steep hill in the middle of summer the temp reached 275 and just kept on chugging... when I reached the top and started coasting down the other side expecting at any moment for the old motor to blow up, but no the temps started dropping and before long was running normal with no issues found. It was the higher boiling temp. of the coolant that saved my bacon. Off road bikers been using EVANS coolant for a long time so it is proven technology.

Digitek14

UPDATE:
Swapped the radiator cap and replaced the coolant with Mopar HOAT. Flushed/filled it with distilled water and ran several times until it drained clear. Still overheating, even on cool nights now.
Confirmed temps with my scanner and it's still approaching 250°F occasionally, 230°F regularly. Radiator fan is definitely working properly.

Any other ideas as to what this could be? I think the only possibilities left are the water pump and radiator? Is there any test I can do on either?

I have heard some nightmares with head or exhaust issues causing this, but its very odd since sometimes it does not do it at all.

resharp001

Since you swapped coolant again I'd presume you are 100% certain you have that fully burped again. I wouldn't think it was a head issue cuz you'd 1) be losing coolant, and 2) be getting exhaust gases in the coolant. You could do a quick combustion test to give yourself peace of mind on that if you wanted.

Going back to the original post.....did you do the flush and change coolants BECAUSE you were having this issue previously or was this just preventative and you inadvertently introduced the overheating issue?

Depending on that answer I was thinking it's not the craziest thing to think a new thermostat could be bad (if you weren't having an issue before). If that is removed from the equation then it does start to seem more like a circulation issue with WP or a radiator that is restricting flow. If you released tension from the serp belt does the WP pully feel normal?

I think if this were my jeep I'd be more inclined to try another thermostat just due to ease of swapping and not much money to try prior to dealing with the radiator or WP. I can't think of it being anything else if you're not losing coolant somewhere.