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Hey guys, I have a 13 jku rubicon with 135k miles, 4” lift on 37’s, winch, etc. been fighting an overheating issue for a few months now. Only overheats on high load on the freeway so going up a grade and going 75+ mph. Or going up a winding steep grade going 45+. Temps will creep all the way up to 250 in some instances. It never gets above normal operating temps while idling or when driving around town or when driving on flat freeway. I have been running 37’s with 3.73 gearing for about 1.5 years now. I think It started around 7 months ago or so around when I went from 37/12.5/17 general grabbers to 37/13.5/17 nitto extreme terrains with a lot more tread. I had replaced my t stat about 2 months before that. When I was driving up the mountain one day I noticed my temps started to rise and I could smell coolant smell. Went home and found my water pump had dried coolant around it. Had a friend come over and check it out and he found my radiator also had a small leak. I replaced the radiator and water pump and it was still overheating under high load. I put a vented hood on it and that didn’t help. I replaced the t stat again and new radiator cap and thought that had fixed it, nope. So then I figured it had to be the gearing was causing the engine to work too hard. So I just finished a gear swap to 4.88’s and I drive it on the freeway last night and hit a grade and it started to overheat again while going 75 and at like 5k rpms. I’m at the end of my rope with trying to figure out what it could be. Any body have any ideas? Could it be tires? I didn’t think it could be head gasket since it never gets hot any other time. Thanks for your help.
 

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How hot is overheating? Gauge just moving off of center? Engine going into limp mode? These engines run hot. And the more aggressive tires might not help.
 

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How hot is overheating? Gauge just moving off of center? Engine going into limp mode? These engines run hot. And the more aggressive tires might not help.
Temps have crept up to 250. So about at the 3/4 mark of the gauge. Engine has stopped accelerating before when it got that hot and forced itself down to 60 mph
 

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Ok. That's into the overheating.any radiator obstructions? If the only thing you changed is the tires, than the extra resistance is most likely causing your issue.
 

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A bit more detail please.

How long are you holding it at 5000 RPMs? Altitude above sea level? Air temps when it does this? Does it “overheat” under normal driving conditions at RPMs below 3500?

240 - 250 degrees is not uncommon for a JK held at high RPMs for an extended period. Especially if you’re at higher altitude climbing a hill.
 

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A bit more detail please.

How long are you holding it at 5000 RPMs? Altitude above sea level? Air temps when it does this? Does it “overheat” under normal driving conditions at RPMs below 3500?

240 - 250 degrees is not uncommon for a JK held at high RPMs for an extended period. Especially if you’re at higher altitude climbing a hill.
I live in Sheridan Wyoming, about 3500ft. Air temps could be 10-55 deg. Never overheats any other time only at 5k rpm going 75 up a grade Or going 50+ up Into the mountains. Usually held 5k rpms for 30secs or so.
Found That there was coolant running down the back of the block and bell housing after pressurizing the system so was able to see where the leak is coming from, there is an oil cooler filter located underneath the intake. Replaced that and now no more leak, but still overheating at 5k rpm going 75 mph up a grade on the freeway.... I’m stumped
 

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I live in Sheridan Wyoming, about 3500ft. Air temps could be 10-55 deg. Never overheats any other time only at 5k rpm going 75 up a grade Or going 50+ up Into the mountains. Usually held 5k rpms for 30secs or so.
Found That there was coolant running down the back of the block and bell housing after pressurizing the system so was able to see where the leak is coming from, there is an oil cooler filter located underneath the intake. Replaced that and now no more leak, but still overheating at 5k rpm going 75 mph up a grade on the freeway.... I’m stumped

Do yo have anything blocking flow to the radiator? Winch, grill insert, bumper with light on it? If so remove them as best you can, key is to get good non blocked airflow. On my jeep I added a small light bar above the winch.. and this alone with enough to divert the air and give me 10 degree in crease in temps going up hills. Took it off and suck the winch down into the frame rails and runs nice and cool now.
 

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I would check the thermostat , simple to do . Put it in a pan of water on the stove and see if it is opening up at the right temp, cheap to check.....Wes
 

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I have determined the thermostat in our Jeep JKs is too small and never opens far enough. Secondary Temp gauges make us watch closely making us aware of temps all the time then we start tripping.
I fixed my JK by drilling 3- 3/16" holes in the thermostat mounting surface This does 2 things it burps locked air and increase flow. You will throw a code if you don't let your rig warmup for 5 minutes in the winter. A
3.6 At high rpm can not pass enough fluid through the tiny half opened thermostat.
I did reset fan to come on low above 190 medium 200 high 220. No more over heating for me.
Oh and no JK is safe to drive above 65 mph.
 

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I'd swap out the t-stat again, could be a bad one. If you have done all that work, that is probably the culprit.
 

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Discussion Starter #12
I have determined the thermostat in our Jeep JKs is too small and never opens far enough. Secondary Temp gauges make us watch closely making us aware of temps all the time then we start tripping.
I fixed my JK by drilling 3- 3/16" holes in the thermostat mounting surface This does 2 things it burps locked air and increase flow. You will throw a code if you don't let your rig warmup for 5 minutes in the winter. A
3.6 At high rpm can not pass enough fluid through the tiny half opened thermostat.
I did reset fan to come on low above 190 medium 200 high 220. No more over heating for me.
Oh and no JK is safe to drive above 65 mph.
Where did you drill those holes? Through the mounting surface of where it mounts to the block? Wouldn’t that be an opportunity for leaks?
Also, how did you change the fan settings?
 

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If you change the thermostat again, suggest you go with OEM. I understand (but can’t personally verify) that some aftermarket JK thermostats do not always open fully and tend to restrict flow.

You have a Jeep that doesn’t overheat under any typical driving conditions so your problem is more of an outlier. The engine, under highest load only, is generating more heat than the cooling system can get rid of. Check thermostat, make sure the radiator surface is clean with no aerodynamic obstructions, confirm no air in the cooling system.

Include a picture of the front end with your winch.
 

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Used HP tuners for fan setting.
Thermo. Remove the thermo from plastic neck.
Outside the bulb you see 1/2" wide surface creating a circle that mounts the thermo.
In that outside edge I drill 3 holes.
For years old mechanics would drill one small hole just to prevent air lock. I drilled 3.
 
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