MJ Comanche Tech. Stock & Modified. MJ (86-92)
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Yet Another Overheating MJ

Old 06-01-2015, 04:52 PM
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Default Yet Another Overheating MJ

So I recently purchased a 1988 MJ 4x4 Automatic AW4 with the "closed", "pressurized", "radiator cap-less" whatever you want to call it cooling system. Drove it on the highway for quite a ways after I bought it. Everything was peachy and I was grinning from ear to ear since my first car was a 1999 Cherokee (I still have it as it's my DD). Ever since I knew these trucks existed I wanted to own one. I finally saved up some cash and I bought it. I drove it to school a few days later and the thing puked coolant all over the ground. I limped it back to my house with the heater on full blast. This time no puking and no boiling of coolant in overflow tank. It now overheats at idle within 15 minutes. BUT here is the plot twist: PO had put an inline radiator cap on the upper radiator hose with a smaller hose running from the cap which hangs down and is open (a little bit of coolant came out and almost got into my eyes). So now I am sitting here researching everything from "burping the renix's cooling system to "open cooling system" conversions and blown head gaskets/cracked heads. Question is: Could this setup that the PO had put in be letting air into the cooling system? Should I get a new upper radiator hose? I recently bought a mopar thermostat from the dealer and was going to install that this weekend.
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Old 06-01-2015, 07:04 PM
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Got any photos of the set-up?

No need to burp if there's a radiator cap in the upper hose.

The little line from the cap area should go to a coolant recovery tank.

Did it puke all the coolant out that little hose? If so, radiator cap could be bad. They don't last forever.
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Old 06-01-2015, 08:52 PM
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Originally Posted by cruiser54 View Post
Got any photos of the set-up? No need to burp if there's a radiator cap in the upper hose. The little line from the cap area should go to a coolant recovery tank. Did it puke all the coolant out that little hose? If so, radiator cap could be bad. They don't last forever.

I can post a picture tomorrow when it's light out. It's the exact same factory setup except it's got a radiator cap on the upper hose. The little hose coming off the radiator hose cap was just exposed and not attached to any bottle or anything. I thought it was very odd. The coolant puked out of the coolant recovery tank.
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Old 06-01-2015, 09:31 PM
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Originally Posted by 1999FLXJ View Post
I can post a picture tomorrow when it's light out. It's the exact same factory setup except it's got a radiator cap on the upper hose. The little hose coming off the radiator hose cap was just exposed and not attached to any bottle or anything. I thought it was very odd. The coolant puked out of the coolant recovery tank.
What recovery tank?
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Old 06-01-2015, 10:16 PM
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Originally Posted by cruiser54 View Post
What recovery tank?
The overflow bottle on the passenger side. It puked coolant out from that bottle.
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Old 06-01-2015, 11:22 PM
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Originally Posted by 1999FLXJ View Post
The overflow bottle on the passenger side. It puked coolant out from that bottle.
I'm confused and need pics.
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Old 06-02-2015, 08:05 PM
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Default Yet Another Overheating MJ

Originally Posted by cruiser54 View Post
I'm confused and need pics.
Same here.
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Old 06-02-2015, 08:49 PM
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What I have observed so far is in order to get around EPA Emission Standards they decided these things should run at just under the boiling point of 212 degrees.

Please keep in mind that 210 at the gauge does not mean that this is what the Engine is truly sharing with the liquid at the Cylinder Walls. I have found that It is imperative to have the proper 50/50 coolant mix to prevent descrete undetectable "Foaming" at the Cylinder Walls.

Cylinders will not cool when boiling in contact with a GAS, This System relies on Contact with a LIQUID. From What I have seen here in the Az Desert so far is this thing is right on "The edge of the Boiling Point" all the time.

Check list to start with would be:
1. Coolant mix ratio
2. Mechanical Fan Clutch
3. Electric Aux Fan kick in Temp (I am going to over ride mine with a switch to the Relay so I can get a jump on it when towing my Trailer)
4. Is the System Cap Truly "Closed" and tight so that it pulls the coolant from the overflow Reservoir everytime it Cycles through a "Cool Down"

These are the basic checks that could mess you up with even though you have a normal system. From here you start looking into more serious issues.

Last edited by Bugout4x4; 06-02-2015 at 08:54 PM.
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Old 06-03-2015, 01:16 PM
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Here are some pics of my setup. My upper radiator hose gets very hot when the gauge reads 230, but my lower radiator hose is not hot at all, not even the slightest. Yet Another Overheating MJ-img_3106.jpg

Yet Another Overheating MJ-img_3110.jpg


I pulled the little hose from the radiator cap from where it was. It just hangs down pointed at the ground.

Also I notice that my electric fan is not coming on when I get above 210 degrees. But then I saw that there is not temp switch for the electric fan??? The radiator is just capped off with some plastic piece. Tomorrow I will be installing a new water pump and thermostat. Will update after I complete that.
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Old 06-03-2015, 02:26 PM
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What a hodge-podge.

You can put a thermal switch in your radiator for the fan.

You can eliminate the pressure bottle also. Then, add a coolant recovery bottle to run the hose from the filler cap vent.
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Old 06-04-2015, 08:20 AM
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you say you felt both the top and bottom hoses the real question is not how hot they are but when you felt the bottom did you squeeze it and see if there was liquid in it? The upper takes heated coolant away from the motor and bottom is going back to the motor after it has been cooled. I believe if the thermostat is stuck closed you wont be getting much water going through that bottom hose and certainly if you get to those temps the system should be fully circulating. Just wierd cause most thermostats these days when they fail they fail stuck open unless jeep made thermo in the 80s that failed closed. I hope this is accurate please correct me if im not. Im anxious to see the outcome of replacing that thermostat.
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Old 06-04-2015, 08:22 AM
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Originally Posted by Scott M View Post
you say you felt both the top and bottom hoses the real question is not how hot they are but when you felt the bottom did you squeeze it and see if there was liquid in it? The upper takes heated coolant away from the motor and bottom is going back to the motor after it has been cooled. I believe if the thermostat is stuck closed you wont be getting much water going through that bottom hose and certainly if you get to those temps the system should be fully circulating. Just wierd cause most thermostats these days when they fail they fail stuck open unless jeep made thermo in the 80s that failed closed. I hope this is accurate please correct me if im not. Im anxious to see the outcome of replacing that thermostat.
Only "fail-safe" thermostat I'm aware of is MotoRad.

The stats used in the 80s and even now from the dealer are not fail-safe.

You must be young.
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Old 06-04-2015, 09:12 AM
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I can't count how many "Brand New" Thermostats I have installed over the years that were bad right out of the box. Some would randomly stick open while others would randomly stick closed. some would open as they are supposed to but would not open enough for proper flow. Most times there was no real "Pattern" to it and hard to diagnose.
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Old 06-04-2015, 09:24 AM
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52028186 from the dealer is the only one that I've come to rely on after changing 6 thermostats among our three Jeeps.

I live where it gets down to 0* in winter and 100* in summer.

At 20* outside temp, I drive 3 miles and my heat is working...

At 100*, my electric fan only comes on at idle.
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Old 06-04-2015, 04:06 PM
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Update:

Didn't have class today so I replaced the thermostat with an OEM 195 degree Mopar one. Pulled the old one out (180 degree t-stat by the way ). I put it in boiling water and it opened. Threw the new one, started it up and the MJ climbed to around 230 but then back down to around 200 and stayed there! But of course, it went right back up to 230+ and then I got angry/sad lol. Might try burping it again after it cools down.
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