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It's been discussed a millions time so what's a million and 1 more

The current mopar thermostat has a little wobble valve in it, coolant forces it closed but air can escape

Mopar_52028186AC.sized.jpg



I always buy dealer t-stat for this reason. If I am forced to buy a parts store t-stat then I drill a hole as mentioned above. Perhaps wrap your exhaust manifold with wrap to keep some heat out.

Four jeeps In counting I’ve yet to hold 210+ temps for more than a moment and I am using stock spec cooling system.


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I understand why XJs have issues with heat, engine bay doesn't breath, exhaust manifold placement, too small of a radiator etc. I don't like running at or over 200* at all. I have a newer rebuilt motor also with a new radiator. So My plan is to run a180* stat with a burp hole drilled in it, new housing and temp sensor with a bypass cut in the gasket, eliminate the clutch fan and go s-blade efan on the passenger side which I've adapted to a new stock fan shroud and a newer stock replacement fan on the driver side. I'll run a 180* stat inline on the heater hose for the pass Efan and my other fan is already on a switch. Hopefully this will keep it 190* or cooler. Air trapped in the coolant system seems to be a problem with the Xj.

That's a lot of work. There are other things you can/should do instead of trying to re-invent the wheel. Most of us here find that choosing a good, not a bargain brand radiator is 75% of the solution to solving high engine temperature. I do agree that the XJ is deficient when it come to the size of the radiator. However, I believe that a good radiator as well as other components of the cooling system (OEM) is more than enough to keep high temperature in check.

My XJ have a 13 year old radiator and that leaks ( it have a 7 psi cap for more than a year), dual e-fan with everything else OEM including the thermostat. I drive from North of Austin to San Antonio in stop and go traffic with the AC on high and my XJ do not get above the 210 mark in stop and go traffic encountered in the construction zone on I-35.

Is your 4.0 running lean? Is the catalytic converter blocked? Gears verses tire size? AW4 transmission or manual shift? Sorry, I did not read the post in its entirety. Many have thrown parts at their XJ in an attempt to bring the cooling system temperature down to later find out that (1) they purchase inferior parts or (2) the problem is somewhere not related to the radiator and other parts related directly to the cooling system.

Since I started on NAXJA, there have been probably hundreds of threads relating to the efficient cooling of the 4.0L . Most have the problem solved by using direct OEM parts and/or good after market parts.
 
It's not that much work, it's all easy stuff. It's got the aw4 and a external cooler is on the list also. It has a new stock replacement radiator, and a new water pump. Both done by the previous owner when the motor was rebuilt. I'm just not a fan of running any engine that hot (over 190). I come from the muscle car/street racing world, I like low operating temps, especially with a flat tappet cam, it's a personal thing. This is my 3rd 4.0 powered jeep, I love em, the factory just runs them too hot for my liking. If I can get it to run 180 on warm days with the AC on I'll be a happy camper
 
That's understandable, but several people have reported degraded fuel economy when running 180 degree thermostats, seems like the 4.0 is just happiest at 195...
 
****UPDATE****
Took some advice and dumped the coolant out and did a 50/50 mix. Seemed to run cooler. Az has been 105-110° consistently lately. I'm starting to suspect ac might be my issue.
Early mornings to work are nice and cool so ac is not necessary. The jeep runs about 190° at highway speeds. After work it's pretty hot and the Ac is on full blast. There is a atech of highway where I live that turns in to a parking lot with stop and go traffic for a couple of miles. The jeep will eventually hit 220° and the ac must be turned off. Once off and moving at highway speed she cools off back down to 210°.
This weekend I headed north to escape the heat of the valley. There were many steep hill climbs to which the Ac had to be turned off do to increased engine heat. However on the way home I left the Ac on through most of the trip back. Once I pulled in to garage and shut her down I noticed that she boiled over and completely filled the over flow tank.
Again, all cooling components have been replaced except the radiator. Which is a duel core all aluminum CSF radiator that is about 3 or 4 years old.
Could it be the Ac compressor over working the motor?
 
Not likely, is your Efan coming on when you turn the AC on? It should. If not start with fuse, relay, etc. to isolate the trouble. Without AC On it should come on at 215*, but as soon as you turn your AC on the AC clutch should engage and your e-fan should come on simultaneously.
 
Not likely, is your Efan coming on when you turn the AC on? It should. If not start with fuse, relay, etc. to isolate the trouble. Without AC On it should come on at 215*, but as soon as you turn your AC on the AC clutch should engage and your e-fan should come on simultaneously.

Not on a 2001 — fan will only run at 215 or whatever the actual setting is — AC will not turn on the fan on 2000 and 2001

Now on my 96 the fan runs whenever the ac clutch engages

It does sound like you need to be certain your Efan is operating — you can test by jumping terminal 30 to terminal 87 where the relay mounts in the PDC — I would suggest using a fused jumper wire
 
Add a switch to turn on your Efan there's a write up in the FAQ section, that way when ever your AC is on you can turn it on before it gets too hot. May want to consider an upgrade on your clutch fan to the ZJ clutch
 
The efan comes on at 210 as well as when I turn on the Ac. All of this was replaced with new parts when the new motor was dropped in. Even though am not using the heater during this time of year. Would a plugged heater core cause this? :banghead:
 
Every time I have the radiator out of the car, I take a garden hose and blast the AC condenser clean from the rear (engine compartment side). Then blast it from the front.
Repeat a few times. It's always good for lowering the engine temp a few degrees.
 
Boiling with a 50/50 mix is 223*. See what I'm saying now? The factory runs them too hot for emissions reasons, run a cooler thermostat and put your efan on a switch , I used my rear wiper switch( deleted the rear wiper) Ran today in 93* with the AC on in 70% humidity. Ice cold AC inside, max operating temp was 210 or less in stock trim( stock fans, stock shrouds, 50/50 mix, newer stock radiador, Not close to overheating, but too hot for me. I have a new 180 stat, gasket, housing, sensor, and s-blade efan to replace the stock clutch fan coming, and I'll do a write up on it.
 
The efan comes on at 210 as well as when I turn on the Ac. All of this was replaced with new parts when the new motor was dropped in. Even though am not using the heater during this time of year. Would a plugged heater core cause this? :banghead:

Interesting that you say your efan comes on with the AC compressor on a 2001 — it shouldn’t

A plugged heater core certainly won’t help — you could bypass the heater core by just connecting the hoses with some copper pipe and a couple of elbows

Have you tried flushing the heater core?
 
OP, I've been here for 57 yrs and have had very few issues with temp unless something is wrong. A lot of those "so called" hi-flow water pumps are just that. The heater core is part of the cooling system as the later models flow all the time so that could be a problem. Buying or making a ported/freeflow thermostat housing is a good idea.

When I built my stroker I had B+G custom tune my PCM and had them set the E-fan temp for 200*.
 
Digging around a little it appears you can possibly get the pcm reprogrammed at a dealer for the Efan to come on with the AC on 2000-01's, and possibly at no charge. No guarantee on the freebie part. Almost sound like there may be a TSB on this.
 
Two things to also think about, the AW4 with an inline-six has a suggested operating range of between 125-176 degrees. Most other Jeep autos (727, th400, 42rle) need to run between 150-200 degrees. And non synthetic, conventional motor oil starts to break down at temps over 230. So when your driving your XJ around in extreme heat coolant temp shouldn't be your only concern.
 
Ok. I think I'm going to buy a new heater core and replace this fall. In the mean time I'm going to bypass the hoses. Sounds like this could help not only with the cooling issue but to help determine if it is in fact the core itself. Since all have the dash ripped out might as well replace the evap as well.
If all else fails I'm going to run without a t-stat. Ireally though replacing all the cooling components and motor would have fixed this. Forgot to mention that I also added hood vents too.
This is so damn frustrating. Thanks for all your help and really useful knowledge!
 
It's pretty easy to set up and flush the heater core -- Use a harbor freight 12 volt pump and a bucket of hot water with your radiator flush chemical of choice -- rig up some hoses and pump the cleaner thru -- be sure to swap directions and run it backwards too -- you wont believe the crap that will come out
 
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