Bubbling in reservoir

Something that happened today scared the living s**t out of me.This is regarding my childhood Xj which is a 99 4.0 with 300k. It has not been my daily for about a year now so I only drive it in nice weather such as the weather we had tonight, dry and warm. When I arrived home from a 30 or so minute cruise and shut the engine off I heard gurgling from the coolant reservoir which lasted for 3-4 seconds. I am a little confused at how this is possible because I gave it a new coolant cap with the last oil change, coolant is one year old and I bled the system a few months ago. If the water pump was not functioning I would have been able to tell during the drive not to mention when I was bleeding the system. This vehicle has always been well maintained. No gunk in the coolant ect. I am at a loss as to exactly why im getting that gurgling.

Air in the system some place, bad hose, heater core, freeze plug., water pump.
Worst case is bad head gasket.


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The exhaust is good so definitely not head gasket although I will still do the test since I have the dye for it. Hoses are new so freeze plug may be the culprit. I honestly completely forgot about plugs I am still focused on how relieved I am that I didnt just ruin the engine

These engines can take quite a lot of abuse before the truly overheat.

With new hoses check the clamps are still tight.

My next biggest concern would be the heater core. Far as hoses and freeze plugs pin hole leaks can be hard to find, can be a issue even with the radiator.

My guess is just about anything else before headgasket. BTW those test only really work if the exhaust contamination is really bad. You can get very small seepage that will leave virtually no sign.

You said it's a 99... I'm not sure but probably need to make sure it's not got the 0331 cylinder head, I believe only the 00-01 jeeps had them and those are prone to cracking.

Yours being a 1999 motor should be a casting number of 0630, which is a good one.





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Yes it does have an 0630 casting number. The radiator is one year old and since the coolant has always been changed while green my heater blows potently hot air leaving me to believe that the heater core is not malfunctioning. Do you know of any way to test it?
Thanks again for the help

Not sure on the heater core testing, when mine go its usually unmistakable, either on the floor or pouring out of the ac drain .


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I have had coolant caps tested at the counter when I bought them and 2 out of three were bad. Have it pressure tested. Check the hose and radiator overflow nipple to the coolant cap, it may drawing in air over night when the coolant coolant cools down and draws a vacuum to suck coolant from the over flow bottle back to the radiator.

I had a vacuum leak at high rpms on the inlet to my water pump once at the water pump gasket on the suction side drove me nuts for years. It never leaked coolant, acted like head gasket leak but was not. It would suck in air on the water pump inlet side of the pump where the pump sealed against the block. I have heard of water pump shaft seals doing this too. Look between the radiator outlet side and the coolant pump inlet where high rpms create the lowest pressure in the cooling system.

Head gasket may not leak at idle during a block test, may need to push the rpms way up to see the leak in a block test?

Good luck!!!

Thank you both so much! I will certainly get to trouble shooting. When I get out of college I am going to spend my entire first years salary just to restore this Jeep. It is my most prized possession but I definitely dont want it to sit for a few years because of this so I am definitely tackling this. How did you get the store to test the radiator caps? I bought mine from autozone its just the duralast or whatever one. I would definitely go back and get another that is tested for good measure

If the engine wasn't overheating, I wouldn't worry about it.
Probably just a burp from either the radiator cap or the
thermostat. Something I do when installing a new thermostat
is to drill a 1/8" hole in the housing. This allows pressure to
equalize when the thermostat is closed. Many thermostats
now have this feature.

alexgalexg said:

Thank you both so much! I will certainly get to trouble shooting. When I get out of college I am going to spend my entire first years salary just to restore this Jeep. It is my most prized possession but I definitely dont want it to sit for a few years because of this so I am definitely tackling this. How did you get the store to test the radiator caps? I bought mine from autozone its just the duralast or whatever one. I would definitely go back and get another that is tested for good measure

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I'm surprised how many XJ are multi generational vehicles. I've got a ten year old who wants us to give him our 92 XJ. His mother is giving me looks like don't you dare cause she wants him in something slightly safer with air bags. He hears this and says he'll have two jeeps then; one in high school and this one when he is an adult so he can restore it. No joke. He doesn't like that it needs a paint job and fact the I've replaced so many original parts like the seats and radio. He was happy when I told him I've still got the original radio.
I already have the boys help me working on it , just a few tasks they can do. Before I knew it he got possessive over it.

One of our tasks (admittedly a few years off so the boys can grow into it) was going to be a thirty years rebuild of the engine, Compression looked weak in a few cylinders, after a few cleaning tricks, rotella oil, and a can of restore it looks like the original 300k engine ( & transmission, front and rear axels original too) isn't going to quit any time soon.
But I also do a lot of preventive maintenance in too.



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Last summer I was looking for a daily driver to replace my 99. It has always been in perfect mechanical order because we took such good care of it. When it became mine years ago I only maintained it to an even higher level of meticulous and preventative maintenance however 300k of New England has taken its toll on the body. I found a barn find 98 Xj that the guy sold me for $500 (he didnt know what he had) and all it needed was an engine. Drove up to NY and got an engine. Rebuilt most of the truck as well as the engine and now I have a mint Xj that is definitely worth something due to being in such good shape plus all the performance parts I put into it. Whole project only cost me a few thousand. So if my love for my 99 got me to do all that just to save it so I can restore it after college then I can definitely see why he likes yours so much!

Thanks for the advice about the hole. I will definitely look into that option and see if it changes things running on empty

alexgalexg said:

How did you get the store to test the radiator caps? I bought mine from autozone its just the duralast or whatever one. I would definitely go back and get another that is tested for good measure

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They have free rental tools. Just make them bring out the radiator-engine block coolant system-radiator cap tester kit, and use it at the counter, go through their caps tell one opens at the listed pressure on the cap. You can also rent it (free, just make a security deposit) and use it to pump pressure in the cooling system and see if it holds pressure for 60 minutes and watch for any leaks.

I know its crazy, jeeps run in my family and especially xj's. I'm 16 and just bought a 99 white xj

Green Mesa XJ said:

These engines can take quite a lot of abuse before the truly overheat.

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^This.^ I once had a radiator cap that would leak coolant everywhere once the engine got hot. I tried to keep an eye on it until I could fix it, but there were points where I was probably driving around with 3/4 or 2/3 of the amount of coolant I SHOULD'VE had in the system. No overheating, even on 20 or 30 minute drives.

One buddy of mine worked as a dealership service tech for 8 years and had the distasteful pleasure of being around during Cash for Clunkers. He said their usual SOP for destroying the cars was to jack them up, take off all the wheels and tires, drain the oil, start them up, put a brick on the accelerator pedal, and come back in an hour. He said every 4.0 and 5.2/318 they did this to was still running after an hour. At that point, they'd fill the oil system up with the sodium silicate solution they had (imagine water mixed with sand) and repeat the process. Apparently, some of those tough little bastards would still run for another 10 or 15 minutes...

Next time you go for a drive, pop the reservoir cap off while the engine is running. If you have a continual stream of bubbles, you probably have compression blowing past the radiator cap.

I hear gargling everytime in turn mine off, no leaks and no overheating though....thoughts?


87' XJ Wagoneer I-6 AW4 NP242

kyle_inreallife said:

I hear gargling everytime in turn mine off, no leaks and no overheating though....thoughts?


87' XJ Wagoneer I-6 AW4 NP242

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That can just be the T-Stat, exhaust manifold and oil dripping back into the pan noises. Been listing to those for 12 years, now at 290,000 miles

start with a radiator flush.
and when you are done
you need to fill it on an incline.
let it run for about 30mins on said incline.
you want the radiator to be higher than the engine if that makes sense.
these 4.0s are not designed well. in the cooling department.
they can and will get air bubbles in the head and thermostat.
I learned you can avoid 90% of these issues by filling with a hose from the heater core outlet tube. it will stop most of the air bubbles from building.
it will fill the back of the head first. and on that incline it should push the bubbles out to the front.

I hope this info helps others.
It originally came form here. I do not remember who told me this.