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Overheating After Multiple Hours of Work


NAXJA Forum User
So I have had this jeep for about a year now and have put multiple hours into fixing it up. I have done some longer trips with it but it has been giving me a lot of issues with heating. It does currently have 32.5" tires on stock gearing which I know doesn't help. But even taking it very easy around town or on the highway it will still kick over the 220 mark and say check gauges (idiot gauge). Sometimes it will hit 260 degrees on hills or if I start to push past 60 mph. I have found that driving in 3 with heat on does help but not very comfortable in the heat. I have had a shop look at it and pressure checked the cooling system with no issues. I am at the point of just wanting to replace the entire motor. It does not boil over when the temp hits 260 which I don't understand with all of the work put into it. The temperature does not seem to be running as high as a temperature gun pointed at the thermostat housing. I inspected the cylinder walls but there might be a pin hole or something I am missing when it starts running hotter. It is a building effect on the heat. Once it starts it does not want to cool down really at all. I will sometimes put the jeep in natural and spin up the rpms to cool it down to the 220 mark.

2000 Jeep XJ
4.0 I6 : 210,000 MI
Automatic 4x4 Sport

Recent Work Done:
Flushed and solvent cleaned entire cooling system 5 times.
New Head Gasket
Magnafluxed Head: No Issues
Re-machined Head
New Mishimoto 3 core aluminum radiator
New Mishimoto silicone houses (spring placed in lower house)
High Flow Water Pump
High Flow 195 Thermostat and housing: boil checked
OEM Mopar Temperature Sensor with new pig tail connector wiring
New Fan Clutch and Electric Fan
New Radiator Cap

Hoping for some good knowledge to get dropped on me. Thank you in advance!
I heard the mishimoto radiators sometimes had a problem with the way the core was cut restricting the water. Also might try a standard water pump, the high flow may be moving the coolant too fast. Other than that I would say try a stant superstat, that's the only one I have ever gotten to regulate temperature correctly.
32's on stock gears is generating lots of unnecessary heat. As suggested, a Stant 195* or genuine Jeep 195* thermostat and a standard flow water pump.

A transmission cooler, an oil cooler, and hood vents will help heat escape, but correct gearing is probably the better solution.
As Tim said, getting regeared will help a lot with all of the heat generation, but we all know this is not cheap. However, it is definitely worth installing a transmission cooler as he mentioned. Like the 4.0 itself, the AW4 can become a furnace, even to the point where it can blow the motor due to the extremely hot transmission fluid passing through the same radiator the motor uses.
I will definitely try switching back to a standard water pump and give one of the other thermostats a try. I plan on doing gearing at some point within the next year but I also don't want to spend the money on it if I can't even get the motor from overheating. Kind of a sticky situation. Have any of you guys seen the CPU misread the temperature?
Do you guys have any recommendations of which transmission coolers that you would recommend as well?

I was looking at going this route:
Hayden Automotive 516 High Performance Transmission Cooler

Also the main reason I have been holding off on the re-gear is if I do the re-gear I would want to upgrade to lockers and upgrade the axels. But might just be better to do it one step at a time for the overall cost and to get it running reliably.
Here's what I did to my 2000XJ to eliminate my heating problems;
1. Removed head, had it pressure-tested, Magnafluxed, re-surfaced, mildly ported the head, re-installed head w/new gasket and new head bolts. IIRC, ARP head bolts.
2. Installed trans cooler; serpentine TUBE-type and NOT the newer type that has very restrictive clog -prone flow passages that increase coolant flow velocity to increase Reynolds Number (i.e. a technique to reduce heat-exchanger size).
3. Changed original thermostat housing to newer designed housing that reduces coolant flow restriction.
4. Changed radiator; IIRC, a 3-row .
5. Replaced standard "idiot light" dash to gage indicators and replaced temperature sensors.
6. Installed new standard sized water pump and standard thermostat.

A final note, I concluded that the "one-use and replace" standard head bolts, when exposed to hot-spots/high temperatures in the head, can yield and cause gasket-venting and MUST be replaced. Therefore, for me, the high strength ARP head bolts were the way to go.

This is what I did and I hope this helps.

Best regards,

You do not want a "tube" style, you need to look at "plate and fin" types like the 779.
I run 2 of them, the other one is for the power steering.

Addenda to My Post #8;
7. When I bought my used 00XJ it had an under engine cover which completely sealed the engine bay. I assumed it was factory and discarded so I could increase the air flow through the engine bay.
8. Next I made two (2) sets of 2-hole aluminium spacer blocks (1/2" & 1" thicknesses) to try. The blocks fit between the hood and hood hinges to raise the rear of the hood and increase air flow through the engine bay. Now the air flow through the radiator and the air flow from under the car enters the engine bay to pick-up the exhaust pipe heat and head heat to exit through the raised hood. I've done this to my 88XJ and 00XJ and they have been running well for awhile now.

Best regards,
2000 in California. Do you have the precats? Have they ever been replaced?
You do not want a "tube" style, you need to look at "plate and fin" types like the 779.

I agree with this. While double the price, they are the much more efficient. I do not know much about the 779 but have heard good things about the B&M SuperCooler 70264.
As far as the cats go I assume it is the factory one's. It does not look like it has ever been changed.

I used the old head bolts when I reinstalled the head. Should this be something I should be concerned with?

Maybe this is causing some kind of bypass in-between the 3/4 cylinder.
(It has been pressure checked, but maybe enough when it gets hot to bypass )

To Do List:
Trans Cooler
Standard Water Pump & Thermostat
Hood Risers
Re-gear (eventually)

Are there any good how to guides on switching to a standalone temperature gauge?

Thanks again everyone. I really appreciate all the advice so far!
Some have noted a surge in temps with Hi-Flow water pumps. See Go-Jeeps page. They put in a restrictor.
Also, since you are noting that you get a temp reduction by driving in 3rd, I would wonder about the OD solenoid. I might look at divorcing the Trans cooler from the Radiator with a plate type trans cooler. I'd get a B&M with the barbs integrated. Those types are suppose to act like a thermostat to control trans fluid cooling.
I had a heating issue that I chased for a while that was actually just the stock gauge and sender not displaying the correct temperature. I installed a good autometer gauge and new sender and it read 20 degrees lower than with the stock gauge. Verified by temp gun that the stock gauge was not correct. I would verify that the head, thermostat housing, hoses, etc are actually running hot with a temp gun. It can also show you how well the radiator is cooling, and both sides of the thermostat housing temps and maybe help narrow down where the most heat is being generated.

Also you can usually find stock sized tires used for really cheap. Throw those on and see if it runs cooler with the correct gearing for the tires. A cheaper way to see the effect of gears/tire size than paying for a complete regear right away.
I have checked the temp with a gun and it seems to be lower than what the gauge is outputting. I will definitely be adding a stand alone temperature sensor to the list. Thank you for the reply!
Had a thought, are you pulling your intake air from under hood? A make shift true cold air intake could help. I cut out the front of my box, then part of the rad support in front of it and used some aluminum flashing and Duct tape to make a home grown cai.