Ok, so last night I'm driving home with my son and stop by a small market along the way to pick up a couple things. I leave the car idling while I run inside, and when I came out a couple minutes later, he told me that he thought he heard a low grumbling noise coming from the engine compartment. I listened for it, but heard nothing, so I drove the mile or so to my house.

Upon entering the garage, I did hear this sound he heard, which sounded like coolant boiling. The temp gauge on the dash was right where it is supposed to be - in the middle, but we all know that means nothing on a MINI. I turned the car off, popped the hood, and saw pressurized coolant escaping my reservoir from the cap, and there was coolant splashed all around.

I waited a bit for things to cool down, so when I removed the coolant tank cap I found the reservoir bone dry, and a puddle of coolant on the floor under the car. The puddle was to the rear of the engine, which told me that it was most likely from the tank than from a sudden leak (hoses, water pump, radiator) up front.

It was late, so this morning I put the car on stands and got to checking things out.

First thing I did was to check the oil for the presence of coolant. Clean and right up to the full mark.

Then I filled the reservoir to the max line with fresh coolant and climbed below to watch for dripping. There was nothing, so it seemed on the surface anyway, that I didn't have a burst hose or leaking radiator.

I then started the car, and connected to my OBDII port with Dash Command and watched the engine temp as it ran.

I checked for the presence of bubbles in the coolant reservoir. Was very concerned that it may be a head gasket issue. No bubbles as the engine ran, plus engine oil checked earlier was clean.

I then watched to see if the thermostat was stuck closed, so as the temp climbed, I surveyed the coolant through the bleeder valve on the top radiator hose, at first no flow, but then coolant began to flow strong when the thermostat opened up - right when it was supposed to, at ​around 187°F or so... So the thermostat checks out.

Next was to see if the low-speed fan would turn on as the temp came up even more. About 10-minutes later, at ​221°F, the Lo-Speed Fan came on, then turned off at 213°F - just like it's supposed to. I never got to check the hi-speed fan operation, because the temp never reached ​233°F, the point where it would turn on. So the relay and the lo-speed fan are working correctly.

Now, I'm under the car inspecting the entire underside while the engine is running, looking for anything that looks suspect. I put my hand on the lower radiator hose, and it was hot to the touch - just like it's supposed to be. If the thermostat was stuck closed, that hose would be cool, so more support lent to the fact that my thermostat is good. Plus, everything was dry below, so no leaks. On the topside, I'm looking closely at the thermostat housing and the array of heater hoses. Nothing dripping there. In the cabin, my carpet is dry, so no evidence that the heater core has leaked.

After running the car for about a half-hour, I again checked the oil, with no evidence of coolant.

Now I'm flummoxed...

Any guesses as to what could've caused that sudden boiling/overflowing loss of coolant episode I experienced last night?

 

What type of overflow tank are you using? I had the same problems with my R53, is there coolant splattered on the heat shield under the overflow tank, if so, that is where your problem more than likely is. The plastic tank leaks at the seams which causes that intermittent problem. I changed to an aluminum unit and have not had that problem since.


Before: See the coolant splatter on the heat shield...
https://cimg5.ibsrv.net/gimg/www.nor...2e8136f3e7.jpg

After, New tank.
https://cimg6.ibsrv.net/gimg/www.nor...82b4241667.jpg

 

Brand spanking new OEM.
Coolant was blowing out from the underside of the cap.

I bought the new tank right after my oil cooler failed a month ago, and I had a messy cleanup process. I figured it was easier to get a new tank than to try and clean the interior of the existing tank.

 

Well, not as busy a thread as I had hoped, but I have been diligently trying to solve the issue at hand by reading up on the subject. One of the causes I found was that exhaust gasses transferred to the coolant via a failed head gasket could be a cause of this sudden boil-over issue. So this afternoon, I drove to the local AutoZone and borrowed their block tester kit, which tests for the presence of exhaust gasses in the coolant.

Happy to report that after running the test as directed, the fluid stayed blue (Had it turned yellow, it would've likely meant a failed head gasket), so a negative on that test.

Another potential (and not uncommon) cause of this issue surprised me, and that was failure of the coolant tank cap itself. Supposedly, if the cap does not hold the 13-15 psi of pressure it's supposed to (some failed caps registered as low as 3-5 psi), the coolant will boil prematurely, spewing coolant all over the place. I had a brand-new spare cap, so I've put it into action and hopefully, that's all it is.

Will report back, but hopefully there are others who have something to add on the subject.

 

Did you happen to get to the bottom of this? I hope you are still active on this site.

Looking for your expert suggestion. I have R56. The coolant boiling in overflow tank just happened to me. Yesterday, after the drive home I smell the coolant when I parked. When I open the hood, I see the boiling in the tank and coolant being pressurized out of vent (top) and the bottom hoses of the tank.

First I thought those hoses caused the leak, so I put a new hose tide for the top and reconnect the bottom hose. I test-drove again and when I came back it still did the same (boiling and pressurizing coolant out of top and bottom hoses.

I also just noticed that I could not hear the fan running even after I left the car running after the drive. It's dark and I didn't have a flash light to see if it's actually running, but it is so quiet that i couldn't hear anything. I could hear it running though when I turned on the A/C.

I checked coolant and it doesn't have any oil contaminated. I also checked oil and it checked out ok.

What would be the next thing you would recommend I should check?

Thank you very much for your help.

 

I did solve this, and I apologize for not following up as originally intended.

​It ended up being the expansion tank cap that failed. As soon as I replaced it with a new cap, my mystery coolant boil-over problem went away, and to date has not reoccurred.

To note: For every PSI of pressure the tank holds (the MINI R53 cap holds 14psi), the coolant boiling point raises by 2°F, so if working properly, the coolant can go as high as 240°F (115.5°C) before boiling. If the tank isn't holding pressure, either due to a failed cap or a split in the tank itself, the possibility of overheating raises exponentially.

As to your issue, as long as your fan is working properly and you can rule it out, I'd look carefully at the tank and check for splits or cracks. The coolant that seems to be coming from the hose at the bottom of the tank might just be coolant flowing down from the top, where it's coming out a possibly failed cap, but it might also be a split in the tank that if present, will also cause pressure loss and overheating. That has been an issue with the R53 tanks, not sure about the R56.

It could be the cap itself, but there's no way (that I know of) to test it alone.
If you can rule out a break/split in the tank, I'd replace the cap, then watch it carefully.

 

i had the same issue, and was new to mini at first. i kept filling, and kept overflowing. I didnt know that u had to bleed the coolant in 2 different places. Eventually led to failed head gasket etc. I then learned of 2 different bleed points. i also got a new res cap. it was now good for a long drive, until i heard hissing from the front of the radiator--there was a small hole in the radiator that was leaking coolant, and introducing air into the system

now that the radiator is fixed, and i know how to properly bleed it, its all good.

​​​​​​​my wifes mini started to do the same thing. when cool, i noticed that the res was dry. i filled it at the res and at the coolant hose in the front, brought it to temp, bled in the 2 spots, and its been good for 6 mths now.

 

thanks for all replies. I found a small crack on the vent hose nipple (top) of my expansion tank. Also when it this happened, it appears that coolant also leak from the connector (L shape, don't know what it's called, see the picture from Pelicanparts as a reference) that connects to the bottom part of expansion tank. I looked for a crack at the bottom but didn't find anything. I suspect that the connector may also fail, so I ordered a new one. Unfortunately, they don't sell just the connector and I have to buy whole set (called Coolant Pipe - Return Line). I'll try to replace these two parts first and see. If it continues, I will then replace the cap.

 

Awesome thread for those who need to diagnose coolant issues! How can we give this more visibility?

 

This tech article should also be helpful for coolant tank issues. It's always good to make sure the coolant tank cap is sealing properly as well.

http://www.pelicanparts.com/techarti...eplacement.htm

 

After replacing the bottle and lower hose connector, I'm glad to say that I believe it fixed the boil-over issue. Test-drove for about 30 miles. Monitored the coolant temp. It stayed between 102-105 C. No sign of boil-over.

Thank you for all the info.

 

Excellent thread !!!!

Great info

thanks for sharing !!!