Coolant reservoir boiling?



Driving home other night and van over heated. At the time there was white smoke and the smell of pancakes and maple syrup. Pulled over and waited for engine to cool. At first glance under the hood, the cap from the reservoir was blown open and it’s contents sprayed through out. Radiator was low about a gallon or so. Had it towed home and friend filled the radiator w coolant.(No Burping or Bleeding). At first it appeared the radiator cap was leaking. Bought a new one but when I went to put it on I noticed a very small pin hole in the plastic connector for the coolant reservoir hose. I epoxed the hole and the leak ceased. When I started and ran the van for a while I noticed the reservoir coolant boiling along with some gurgling sounds. there was also leaks present on the floor. Turned the van off, cooled it down, topped of the coolant and started it back up. Reservior still boiled but van was not over heating (yet). Could not detect any leaking this time. Fan worked fine too. took abt 15 to 20 mins more (30ish total) before needle made slight bump up in temp. Still detected no further leaks but coolant in reservoir still boiling. Can air in the system make that happen or is it the gasket? Your thoughts PLEASE!


It is ‘the’ gasket.


“the” like single gasket or “those” gaskets like plural? or is it “them” gaskets? in Arkansas I suppose.


The head gasket.
I suspect that the “boiling” you were seeing in the reservoir wasn’t boiling at all, but was actually combustion gasses being blown into the water jacket and migrating out to the reservoir, where they were coming out as bubbles. That would indicate a blown headgasket.

I’d recommend starting by testing the coolant for the presence of hydrocarbons. If in doubt, I’d run a “leakdown test” on the engine. That’s a test where each cylinder is tested through its spark plug hole for its ability to hold pressure. Failure generally means/confirms a breech in the headgasket, and that would be where the combustion gasses are blowing into the water jacket.


The OP implied that they understood what ‘the’ gasket was. And I am not aware of any gasket failure other than the ‘head’ gasket that will result in the OP’s symptoms.Of course the head could be cracked.but if so that would be noticed when replacing ‘the’ gasket.


Rod, the OP asked for our opinions, not just yours. I found it unclear that the OP actually understood what the “gasket” was. I was adding a bit of clarity to your suggestion. Why are you trying to start an argument?


I wasn’t…


Thanks guys. I suspect it is the head gasket but I hope it can somehow be an air bubble. Will find out soon. Had to drop it at the garage because I don’t have a pressure tool nor combustion test kit so we will see what they say. I am not a mechanic, just a single Mom and knowing what I am looking for HELPS A LOT when I have to bring my car in. Garages tend to try and rip me off far too often. I have studied a lot this past year but I am not an expert- your opinions are helpful & appreciated. Thanks & Merry Christmas


If the cooling system doesn’t hold pressure it can definitely boil like that. Usually when that happens it leaks too. With no leaks, it does sort of sound like a head gasket problem unfortunately. But no need to guess. Sounds like you’ve done the sensible thing and taken it to a shop for proper testing & assessment.

fyi, if you ask around, some auto parts stores will lend you their coolant system pressure tester gadget .


I would have liked to get the tools from the auto store but they stopped rentals or loaners. One reason I consider air v. exhaust in they system is that the steam/smoke occurs as the temp gauge raises and not from the get go. I wish I had seen if the coolant itself was hot to the touch. The bubbling starts when the thermostat opens and the fan turn on. But while I did not touch it I could feel heat emanating from the reservoir tank. I have replaced a valve cover myself but never a head gasket. When the initial over heat happened, I lost a lot of coolant. The cap was bed but when I went to replace that, I found a small hole in the plastic connector leading from the top of the radiator -right under the cap- to the reservoir. I epoxed that and the leak was sealed. New coolant was put in but it was not bled or burped. I THINK there may still be a small leak toward the bottom as the coolant still goes down, a little. If they say it’s the gasket I MIGHT try a sealer first? Thanks for your input


One time my Corolla’s radiator blew its top, due to the radiator cooling fan not coming on like it should in stop and go traffic. I happened to be in San Francisco at the time and thought the steam from the radiator was just SF fog … lol … but eventually I figured it out. Anyway I tried sealing the leaky radiator, worked fora day or two, but always failed. I don’t recommend using sealant to fix cooling system pressure, that sort of fix is too unreliable. Better to just replace the part that is leaking. In my Corolla’s case I replaced the radiator, and the coolant temp activated radiator fan switch, didn’t cost too much, and problem completely solved.


UPDATE: (2008 Kia Sedona V6 - base) Problem: Overheated on freeway. Discovered coolant leak but did not have tools to pressure test system or Combustion Tester to check coolant for exhaulst to indicate blown head gasket. Coolant in reservoir Bubbled & Boiled- brought it to garage for checks) RESULTS: Well, it was a bad radiator cap, cracked (plastic) radiator and a bad thermostat (not working effectively). $700 repair including labor. The coolant in the reservoir was bubbling because air was getting into the system AND boiling because it was overheating as well. Head gasket & Block was good-thank goodness. Just wanted to update in case anyone else had this problem and wanted to know what my results were. Thanks everyone!


The coolant was boiling because the system was unable to hold pressure. The system is intentionally kept at typically about 15psi of pressure (the actual rating is on the radiator cap) in order to raise the boiling point and prevent boilover. A typical engine runs at right about the boiling temperature of water (at 29.92hg, at 23C, etc. etc. etc. To most people it’s just blah blah blah).

I’m seriously happy that it turned out to be just leaks.
Happy motoring.


You are, of course, correct, but this reminds me of something that I used to deal with on a fairly steady basis when I worked in a gas station, back in the late '60s.
My station was on the NJ Turnpike, and we used to get customers from…all over the US. We prided ourselves on excellent service, and every car got a full fluid check, as well as a windshield washing. Upon request, we would also check and correct tire pressure and clean the rear window. (And…no…I rarely received a tip for the extra services…)

Anyway, when somebody from the Deep South pulled into the station, I could usually anticipate what I would find when I lifted the hood. For some reason, it was very commonplace to find the radiator cap opened one notch, so that it couldn’t build up the required pressure in the cooling system. When I would point this out to those customers, the usual response was, “Man, don’t you know that it builds-up a lot of pressure if you tighten it all the way?” :confounded:

The first few times that I encountered this, I tried explaining how pressurization would raise the boiling point of the water in the cooling system in order to curtail overheating, but after seeing that my explanation was…unwanted…I ceased my educational efforts, and just got used to seeing the “southern cars” trailing a plume of steam from their radiator caps.

Of course, these folks also wanted me to refill their radiator, and my standard reply was…“Okay…pull over there for about 20 minutes, and then we can try opening that cap”. They always drove away, rather than wait…


LOL, now THAT was funny!
I guess the school systems don’t teach too much physics in the south either! :joy:


…and they also didn’t teach the concept of ethics, apparently.
I say that because our station was the first one that people would come to, after exiting the bumper-to-bumper traffic in and around the Lincoln Tunnel. It was commonplace for drivers (from EVERY state) to specifically ask for their radiator to be checked, without revealing that their engine was already overheating when they pulled into the station. George–one of my co-workers–damn near had his head blown off by a radiator cap that became airborne (along with a scalding blast of steam) because the driver had–as usual–failed to reveal that his engine was overheating.

After George had to be rushed to the ER for burn injuries, the station manager had signs printed that stated, Absolutely NO radiator checks!


I’ve had severe burn injuries (not from an automobile) and nothing is more painful IMHO.


Glad you got your Kia’s cooling system problem solved OP. Good for you. I’ve always thought from a cost/benefit point of view it would make sense for car owners to replace the radiator cap and thermostat every three years when replacing the coolant. Those two parts live in a very rugged environment and can’t be expected to last forever. But of course, while I’ve thought about it, I’ve never actually done it. … lol . I just replace the coolant every three years and otherwise hope for the best.


LOL, than I guess I’m in good company! I have more than a few things I know I should do… and don’t! :grin: