XJ Coolant Bubbling Out of Reservoir Tank? - JeepForum.com
 
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post #1 of 6 Old 05-12-2019, 04:27 PM Thread Starter
mrkevelev
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XJ Coolant Bubbling Out of Reservoir Tank?

2000 Jeep Cherokee Sport (4.0 L, 4x4). I had my engine idling for about 10 minutes. I then shut it off. About 2 minutes after I turned it off, the coolant began to bubble out of the overflow/reservoir tank. It was fine when the engine was running, and the temperature gauge was sitting right in the middle. I checked all the fluids a few months ago, and haven't had any issues. The only thing that's changed is that the weather has gotten a bit warmer (transitioning from NY winter to spring). Anybody ever seen this before? Also, the thermostat was replaced about 2 years ago, and the coolant level was good. I've been keeping an eye on it periodically since my TUPY head swap last year. So really there haven't been any changes in the last few months.

Thanks!

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post #2 of 6 Old 05-12-2019, 06:46 PM
mukluk
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The first thing that comes to mind would be the radiator cap is old and not holding pressure as it should. Next in line would be you may have some air in the system that needs to be worked out.

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post #3 of 6 Old 05-13-2019, 12:57 PM Thread Starter
mrkevelev
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Mukluk,

I don't understand the part about the pressure cap. If it leaked, wouldn't less air be trapped in the system? Isn't it positive pressure inside the radiator? What about the fact that it occurred only after I shut the engine off?
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post #4 of 6 Old 05-13-2019, 02:00 PM
mukluk
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Increasing the pressure on the cooling system increases the boiling point of the fluid. Stopping the engine also stops the flow of coolant, allowing areas of coolant in contact with the cylinders to absorb more heat as it just sits there (heat soak). If the radiator cap is weak and providing less than the correct pressure rating, it will allow the fluid in these hot spots to boil, producing bubbles and spitting into the overflow tank.

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post #5 of 6 Old 05-14-2019, 03:18 PM Thread Starter
mrkevelev
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Quote:
Originally Posted by mukluk View Post
Increasing the pressure on the cooling system increases the boiling point of the fluid. Stopping the engine also stops the flow of coolant, allowing areas of coolant in contact with the cylinders to absorb more heat as it just sits there (heat soak). If the radiator cap is weak and providing less than the correct pressure rating, it will allow the fluid in these hot spots to boil, producing bubbles and spitting into the overflow tank.
The cap is providing a good seal. I should've mentioned this earlier, but i recently installed a new cap a few months ago (must have forgotten).
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post #6 of 6 Old 05-14-2019, 04:09 PM
mukluk
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The spring rating is what's important. Did you install an 18lb cap?

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boiling , coolant , overflow tank , overheating , temperature

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