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Old 12-01-2018, 05:07 PM
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Coolant Boil

I have an 01 TJ, 4.0 manual transmission Wrangler. I recently noticed the coolant boiling in the reservoir after a drive, but the temp guage registered normal. I read on previous forums that it was most likely the radiator cap needing replacing. I bought a new cap...18lb, and placed it on. The radiator reservoir boiled again after a short drive and actually blew from under the new cap under pressure before i quickly turned it off. Again, normal gauge readings. Any help would he greatly appreciated.

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Old 12-01-2018, 05:34 PM   #2
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Old 12-01-2018, 05:55 PM   #3
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Was the new cap fully seated? It shouldn't boil out from under the cap. Check to see if the radiator is cracked.

If the system is maintaining proper pressure with the new cap and still boiling over, it could be a blown head gasket or cracked head.
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Old 12-01-2018, 06:04 PM
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Thanks for the response. I cant see any cracks in the radiator, but it definitely blew from under the cap like it had too much pressure. Once it cooled i removed it and put it back on to make sure it was seated correctly. Coolant is up to the bottom of the cap. How can i check the head gasket.
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Old 12-01-2018, 09:30 PM   #5
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Make sure the tube to the overflow tank isn't plugged either. What brand cap?
Stant is a good reliable brand for cooling system parts.
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Old 12-01-2018, 10:48 PM
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It is a murray brand cap. I havent seen any signs of water in the oil or white smoke from the exhaust leading me to believe it isnt head gasket related. Could it be thermostay or system flush?
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Old 12-01-2018, 10:51 PM
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I will check the hose tomorrow first thing. Thanks for the suggestion!
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Old 12-02-2018, 01:40 PM   #8
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Seems if the coolant is boiling, then there may be a restriction in the coolant loop that prevents adequate cooling by the radiator. Or possibly the water pump isn't pushing enough volume, or the thermostat is stuck closed. A collapsed radiator hose is a remote possiblilty too.

I replaced the radiator on my 2000 TJ 4.0L because it was beginning to run a little hot. The radiator tubes were nearly plugged with rust and gunk, easily seen through the radiator cap if you drain off a a half gallon of coolant to expose the tops of the tubes.
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Old 12-02-2018, 02:02 PM   #9
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Go to your local parts house and get a coolant test kit for exhaust gases.

If the coolant tests positive most likely a head gasket or cracked head.
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Old 12-02-2018, 04:36 PM   #10
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If the temp gauge is reading normal, it shouldn't be boiling. Normal temp would eliminate stuck thermo or plugged radiator.

That leaves a bad cap or cracked rad bleeding off the pressure or head gasket/head issue.

One more question, is your coolant fresh and the proper ratio of coolant to water?
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Old 12-02-2018, 05:40 PM
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The coolant mixture is good to go, and it had been running fine prior to the last couple of days with no boil issue. If the radiator is cracked and losing pressure would there be a coolant leak or not necessarily? Also, when I cranked it up a few minutes ago, i noticed a small amount of smoke or steam come from in front of the radiator so i quickly turned it off. This appeared seconds after cranking. Could it be coolant evaporating from the previous blowout? I am planning on ordering a pressure test kit and am thinking about a flush just in case it is clogged before trying a new radiator.
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Old 12-02-2018, 06:01 PM   #12
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It sounds like you have a leak somewhere in the radiator.

I think some of the parts stores will loan you a pressure test kit. That's where I'd start looking. The leak may be so small it's not leaking much and it just evaporates.
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Old 12-02-2018, 06:58 PM
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Thanks, i will try and get my hands on a pressure test kit early in the week, and will update yall with the results.
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Old 12-03-2018, 01:07 AM   #14
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Yep kind of sounds like a leak ..
They do make a dye kit to help find leaks, it glows.
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Old 12-03-2018, 05:57 AM   #15
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Could it be a small leak at the hoses or thermostat housing? Seems this happened to me years ago with mine but I don't remember what I did to fix it now.
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Old 12-03-2018, 06:02 AM   #16
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Old 12-04-2018, 05:30 PM
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New development. It appears its an aftermarket fan was attached to the radiator. It had several clips that were attached directly to the back, and it also has what appears to be a temp sensor stuck in the back of the upper corner of the radiator. Is this a bad practice? Should i find a different fan while i have it off?
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Old 12-04-2018, 05:32 PM
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I am in the process of replacing the radiator, so i had to dettach the fan housing already. Most of the clips holding the fan on the back of the radiator had broken or were damaged from vibration and age.
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Old 12-05-2018, 08:35 AM   #19
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Had this issue in my 2002 TJ. Turned out to be the water pump. The back of the impeller had worn through thus not pumping enough fluid but maintained temperature at during speed.
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Old 12-05-2018, 09:56 AM   #20
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I had a similar issue in my 06. From what I could see, it was mostly the radiator cap spring getting weak and not allowing the system to get to adequate pressure to keep the coolant from boiling. It allowed some coolant to escape into the reservoir, and it would sit and gurgle after a long steady climb up a really easy hill. But you can't simply replace a radiator cap without doing the hoses, if they have been installed for a long while. You'll blow a hose sure as hell. So, I decided to do a full service on my cooling system. Water pump was fairly new, but it's evident they had not replaced hoses or the cap when the dealer or PO did the water pump. Flushed the whole system, including the heater core, with distilled water and a bit of radiator flush chemical. That worked well. I replaced the hoses, clamps, thermostat, and the radiator cap, and it worked fine for about 6months, but eventually cracked one of the plastic radiator tanks. Replaced the radiator, and it's been fine ever since.

Cap and thermo are Stant. Hoses are gates. Clamps are a mix of original style and stainless band type, but the originals actually work really well. Radiator is Mopar OEM. No complaints whatsoever.
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Old 12-05-2018, 10:18 AM
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Thanks for the help!
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Old 12-05-2018, 06:15 PM   #22
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a regular flush should be part of "routine" maintenance. cleans the crap out and helps point out any weaknesses.
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Old 12-08-2018, 09:07 PM
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Replaced the radiator and it fixed the problem. Unfortunately the drain plug o ring must have been bad because afer a short test drive i noticed coolant dripping from the plug outlet. I bought a new plug and put it in to keep from losing the rest of my 40$ worth of coolant
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Old 05-27-2019, 05:57 AM   #24
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My 07 LJ 4.0 is doing the same thing. The reserve or overflow plastic tank to the right of the radiator boils and fills with coolant after a ride that gets the engine hot, then when colls seems to drain; short or long ride but especially a long ride... I changed thermostat eventhough the temp gauge never registered hot, burped the cooling system ; topped it off. One difference is that I've noticed a whitish liquid that apparently spews from somewhere near front of engine compartment to the inside hood and outter fender; sort of all over the place. It isn't slick like oil or even radiator fluid but more like whitish water... I'm changing radiator cap today just to see what happens. Note that the thermostat does open allowing coolant to engine - We tested it...any advice indeed greatly appreciated. Trying the cheap way out first... lol.
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Old 05-28-2019, 02:03 PM   #25
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Quote:
Originally Posted by Jeep1965 View Post
My 07 LJ 4.0 is doing the same thing. The reserve or overflow plastic tank to the right of the radiator boils and fills with coolant after a ride that gets the engine hot, then when colls seems to drain; short or long ride but especially a long ride... I changed thermostat eventhough the temp gauge never registered hot, burped the cooling system ; topped it off. One difference is that I've noticed a whitish liquid that apparently spews from somewhere near front of engine compartment to the inside hood and outter fender; sort of all over the place. It isn't slick like oil or even radiator fluid but more like whitish water... I'm changing radiator cap today just to see what happens. Note that the thermostat does open allowing coolant to engine - We tested it...any advice indeed greatly appreciated. Trying the cheap way out first... lol.
A couple recommendations. Don't know what the white water is, but if your coolant is more than a few years old, I'd flush the entire system with distilled water, drain, replace hoses including heater hoses, and put it all back together with the new cap.

A new cap will raise the system pressure. With hoses that are used to the lower pressure of a worn out cap, they will split at the worst possible time. I just replaced my hoses an additional time, because the supposedly high quality hoses I put on it 3 years ago were already starting to balloon and swell.

I also installed clamps on the larger hoses from Clampco. Got them at McMaster Carr. Like these: Standard T-Bolt Band Clamps | Clampco Products

I now have no worries whatsoever of a hose, clamp, radiator, or cap failure. Use Zerex G-05 coolant in the recommended mix with distilled water, and it will get along with all the materials in your cooling system.

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