Coolant temp getting up to 254°

[Halon128]

Problem resolved

About a year ago I replaced my thermostat with a mishimoto thermostat that opens at 180°. After replacing it I was pulling a small trailer at highway speed. The coolant temp gauge was at the 3/4 mark. I slowed down to 55mph and the temp came back down to normal. I chocked it up to the load I was pulling at 75mph in 100° ambient temperature. Fast forward to about a week ago. We had temps of 110° while driving home from work the temp got back up to the 3/4 mark. I was doing 60mph and not pulling a load. I decided to go back to an oem thermostat to see if it would fix it but it’s still happening. I only have this problem in the summer. It’s full of coolant the water pump seems to be circulating the coolant well. I’ve washed out the radiator really well to make sure it wasn’t full of dust and bug guts. Ive owned 2 JKs and never had this kind of issue up until last summer it was fine the previous 4 summers.
Any input would be helpful.

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[chevymitchell]

About a year ago I replaced my thermostat with a mishimoto thermostat that opens at 180°. After replacing it I was pulling a small trailer at highway speed. The coolant temp gauge was at the 3/4 mark. I slowed down to 55mph and the temp came back down to normal. I chocked it up to the load I was pulling at 75mph in 100° ambient temperature. Fast forward to about a week ago. We had temps of 110° while driving home from work the temp got back up to the 3/4 mark. I was doing 60mph and not pulling a load. I decided to go back to an oem thermostat to see if it would fix it but it’s still happening. I only have this problem in the summer. It’s full of coolant the water pump seems to be circulating the coolant well. I’ve washed out the radiator really well to make sure it wasn’t full of dust and bug guts. Ive owned 2 JKs and never had this kind of issue up until last summer it was fine the previous 4 summers.
Any input would be helpful.

Fan working? Any mods to the hood? Fender liners installed? If the coolant isn’t leaking out, then you need to make sure that coolant is circulating properly and that there’s enough vacuum in front of the radiator to cool it down.

Any grill inserts, winch in the way, or lights installed in front of the grill?

Do a coolant flush and fill it with 50/50. Your mixture could be off, as well. Not enough coolant will allow the water to retain more heat.

 

[Halon128]

The fan is spinning as fast as it can. No fender or hood mods. I took the grill insert out when it did this the first time. I do have a winch and was considering it as one of the factors but It’s been on the bumper 4 years. Perhaps a coolant flush or a what it needs. I already use the 50/50 in it. I think I’ll give it a flush and see what happens. Thanks for the response .

 

[mwilk012]

Mix it a little rich on water if you really have to.

 

[Yawnie'sPapa]

I have a TSB from AMC from years ago where they did a study on cooling system performance.
They studied flow rates of various coolant mixes. Cooling systems work best when the coolant flows fast enough.
They found that the highest rate measured in GPM was with a perfect 50/50 mix of coolant and water. Anything else slowed the flow rate (and thus reduces the efficiency of the cooling system)

You mentioned replacing the stat - did the issues start with that? If all else is equal, the stat only sets minimum operating temperature - assuming it's functioning properly.
If it's not fully opening at that temperature then it's obviously a problem

Also make really sure to not mix coolant types - if if takes OAT or has OAT in it, don't use HOAT, if it's IAT, don't use the others and so on. And these days - you can't go by color. I've found each of the types may come in colors that the other types come in. You can't say "it's orange so it's ok". Companies have screwed up the colors. Mixing types can cause gelling of the coolant in some cases.

And of course towing near max capacity at 75 mph - I never tow over 65 if I'm towing half of the rated load or more.

 

[mwilk012]

A marginally lower flow rate is pennies compared to the change in specific heat capacity from antifreeze to water.

50/50 Antifreeze cools 20% poorer than pure water.

Changing the thermostat does nothing in this equation, once you’re paste the opening point it’s just a fixed flow restriction.

 

[Upnarms]

I pull a small off road trailer up the mountains in AZ. Have pulled it in 100+ degree temps. I haven't seen my engine temps get past 232. Seems like there is something that kicks in around that temp, like a thermostat or the fan goes into overdrive, and my temps start dropping into the 220 range. Id imagine the actual thermostat opens up well before 230 so I'm not sure what the vehicle is doing to drop the temps. I do kick off my AC on the big hills when it's over 100 out. 254 seems real high. My last JK would get into the 240s occasionally on the same situations.

I assume you have checked your oil level? My JL consumes a quart during oil change intervals where I tow a lot. That would def contribute

 

[Halon128]

I have a TSB from AMC from years ago where they did a study on cooling system performance.
They studied flow rates of various coolant mixes. Cooling systems work best when the coolant flows fast enough.
They found that the highest rate measured in GPM was with a perfect 50/50 mix of coolant and water. Anything else slowed the flow rate (and thus reduces the efficiency of the cooling system)

You mentioned replacing the stat - did the issues start with that? If all else is equal, the stat only sets minimum operating temperature - assuming it's functioning properly.
If it's not fully opening at that temperature then it's obviously a problem

Also make really sure to not mix coolant types - if if takes OAT or has OAT in it, don't use HOAT, if it's IAT, don't use the others and so on. And these days - you can't go by color. I've found each of the types may come in colors that the other types come in. You can't say "it's orange so it's ok". Companies have screwed up the colors. Mixing types can cause gelling of the coolant in some cases.

And of course towing near max capacity at 75 mph - I never tow over 65 if I'm towing half of the rated load or more.

I pull a small off road trailer up the mountains in AZ. Have pulled it in 100+ degree temps. I haven't seen my engine temps get past 232. Seems like there is something that kicks in around that temp, like a thermostat or the fan goes into overdrive, and my temps start dropping into the 220 range. Id imagine the actual thermostat opens up well before 230 so I'm not sure what the vehicle is doing to drop the temps. I do kick off my AC on the big hills when it's over 100 out. 254 seems real high. My last JK would get into the 240s occasionally on the same situations.

I assume you have checked your oil level? My JL consumes a quart during oil change intervals where I tow a lot. That would def contribute

I have a TSB from AMC from years ago where they did a study on cooling system performance.
They studied flow rates of various coolant mixes. Cooling systems work best when the coolant flows fast enough.
They found that the highest rate measured in GPM was with a perfect 50/50 mix of coolant and water. Anything else slowed the flow rate (and thus reduces the efficiency of the cooling system)

You mentioned replacing the stat - did the issues start with that? If all else is equal, the stat only sets minimum operating temperature - assuming it's functioning properly.
If it's not fully opening at that temperature then it's obviously a problem

Also make really sure to not mix coolant types - if if takes OAT or has OAT in it, don't use HOAT, if it's IAT, don't use the others and so on. And these days - you can't go by color. I've found each of the types may come in colors that the other types come in. You can't say "it's orange so it's ok". Companies have screwed up the colors. Mixing types can cause gelling of the coolant in some cases.

And of course towing near max capacity at 75 mph - I never tow over 65 if I'm towing half of the rated load or more.

Originally I replaced the thermostat because the plastic housing cracked. I decided to go with the misimoto thermostat at that time. It opens at 180° so I figured that would equal better cooling. I’m going to do a flush of the system this evening and replace the coolant with a 30/70 mix of coolant and water. It rarely gets to sub zero temps here in Fort Worth.

 

[Halon128]

I pull a small off road trailer up the mountains in AZ. Have pulled it in 100+ degree temps. I haven't seen my engine temps get past 232. Seems like there is something that kicks in around that temp, like a thermostat or the fan goes into overdrive, and my temps start dropping into the 220 range. Id imagine the actual thermostat opens up well before 230 so I'm not sure what the vehicle is doing to drop the temps. I do kick off my AC on the big hills when it's over 100 out. 254 seems real high. My last JK would get into the 240s occasionally on the same situations.

I assume you have checked your oil level? My JL consumes a quart during oil change intervals where I tow a lot. That would def contribute

I did an oil change a few weeks ago so the level should be good. I’ll recheck that this evening while I’m flushing the radiator. I have a feeling my radiator may be corroding.

 

[mwilk012]

You sure the radiator isn’t full of mud?

(On the outside)

 

[Halon128]

You sure the radiator isn’t full of mud?

(On the outside)

Yes. I wash it out until there’s nothing but clear water comes out. I also rarely play in mud.

 

[Halon128]

Mix it a little rich on water if you really have to.

I flushed the system and did a 30/70 mixture. Driving home today i did 80mph for 10 miles. The gauge stayed right in the middle where it should be. I’m thinking it’s fixed. Thanks for the suggestion.

 

[Halon128]

Fan working? Any mods to the hood? Fender liners installed? If the coolant isn’t leaking out, then you need to make sure that coolant is circulating properly and that there’s enough vacuum in front of the radiator to cool it down.

Any grill inserts, winch in the way, or lights installed in front of the grill?

Do a coolant flush and fill it with 50/50. Your mixture could be off, as well. Not enough coolant will allow the water to retain more heat.

Did a flush last night drove home and the gauge stayed dead center while doing 80mph for about 15 minutes. I think it’s fixed.

 

[azjl#3]

Radiator fins, they clean?

In my JK 2dr rubi, I too once saw 230 239 going up hill AC on at 110f. slowed to 50 and it dropped back to below 230. 230 is right around where hi speed fan kicks in.

Auto tranny? AC on? Ever notice the signs that say turn AC off on hill? It works, will drop temps about 20 degrees.

I added water wetter, and son of a gun, another 7-10 degree reduction in temps doing the same drive under identical conditions. But I still turned on recirc, turned off AC compressor on the hill. <ore power, less heat, coasting down hill, full AC mama!

My JLUR this year? Same conditions, temp gets to 225 doing 60 in 6th or 7th. If I am heavy I still turn AC off, that drops oil temp about 10 degrees. No water wetter yet installed. I plan to do it, if nothing to make winter heater even better.

Tranny temp did not change much.

As stated before, 50/50 best ratio. 100% water is really the best, but then that boils at 212, and about 230 under 15 PSI. 50/50 gets you up to 260 ish.

There is actually a coolant out there that can keep it from boiling over up to 300 degrees. But there are other problems with it. cost being one.

Lastly, dont play with thermostat other than to swap them out, yours may have been stuck, or is stuck. the stock 190-195 is best, your engine is tuned for that, and can cool for that.

Bottom line, reduce heat by slowing and turning off AC.

 

[alphawolff]

For anyone else who has this issue we've seen a lot of comeback overheating issues after replacing the thermostat with aftermarket ones. The OEM one has been horrendously backordered for a few months now so we had to start locating aftermarket ones for p0128 DTCs.

I'd say 1/20 come back for overheating. I'd use OEM whenever possible here.

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