If this were the case wouldn't the coolant level raise immediately and the car not be able to cool itself? I don't see how I could have mixed them up, I have the diagram and they all fit nicely into their own spot, I would need to force one to the wrong location to do it wrong. If the coolant didn't run through the thermostat and the lower radiator hose to the expansion tank once at operating temp I could see this as true.

 

Sorry, buddy, I really wouldn't know what it's like to have it mixed up during the install. But I do recall a post not long ago (this year or late last year) that there's a guy with very similar symptoms after a thermostat housing install. And I vaguely remember that it also caused the coolant to expand in the coolant reservoir. At the end of that thread, the OP simply mentions that problem was addressed after he swapped the two hoses up and everything's back to normal again.

 

 

From all your checks it looks like there's only one thing I can think of: there's a leak/opening somewhere that this supposedly close-loop system has place to escape. And assuming no leaks and no more air bubbles like you mentioned, yea, I would think to replace the expansion tank. More than likely, the cap's overflow mechanism is compromised. See if you can replace just the cap first and see if that does help with the issue.

EDIT: saw your post about having replaced the cap already. You're probably needing to replace the tank now. If that doesn't do the trick, I'm more than sure that there's an exposed opening between your coolant system and the atmosphere, assuming all your hoses and your thermostat housing are all checked out.

If the system remains completely closed, the boil coolant mixture will remain tamed under the level visible in the expansion tank. If there's an opening somewhere, it'll allow the boiled coolant to rise.

Also, you mentioned that you've replaced the thermostat housing prior, There is also one common issue which can easily be unseen: The hose connection under the expansion tank is held by a clip and internal gasket. See if you have a leak under that tank.

 

I really appreciate your taking time to help new out. I have tried a new cap, which is not OEM, but matched the once that was on it before. I was reading a Gen 2 owners manual and it mentioned that the cap should be tightened until it clicks, neither of these caps is designed that way and I'm assuming the OEM cap doesn't either.
I did have a gigantic burping in the system when I restarted the car and maxed the AC to get the coolant to lower as it was near overflow after another test cycle. Made me think of someone shotgunning a beer as the system chugged the coolant out of the expansion tank. This through a code for a relay that I don't remember exactly which one. Likely don't need a new one but ordered it anyway at the autozone. I'm stopping back in there to get the exact relay name today. I'll post more later.

 

When the expansion tank cap closes, you should be able to tell. Though there aren't any distinct clicks or hard stop, you can tell because it's pretty loose to go in the tightening direction until a distinct stop/resistance.

If we're near each other, I'd bring my kit to you so you can pressurize it and see where an opening may exist. But below I bought a kit from ECS-tuning
https://www.ecstuning.com/b-bavarian.../b8800084~bav/

It's what I use when all else fails to locate a leak. If your hoses are correct and nothing is flowing in the wrong direction, then I would think you have a leak somewhere. It's the only explanation left to explain why the boiling coolant would rise instead of being pressed down on its correct level.

There is one other occasion from my past experience with these MINI's that a symptomizes a boiling rise in the expansion tank: A failed or failing coolant pump. But I really doubt that's the case for you. One of those times I remember, the friction pulley failed, and in turn, the water pump wouldn't spin at all. When I open up the hood, the tank is about to blow up, LOL. I really don't think this is your case because your temperature seems to be properly regulated.

 

I wonder if my low speed fan is the issue and maybe this relay is for that which coded? We'll see, the part is labeled a miscellaneous relay, will replace tomorrow. Thanks for your offerto help if local.

 

Chock that up to a bonehead move, my check engine light wad in because I didn't tighten the intake clamps after tearing everything off to inspect my hose connections. Expansion tank it is than.

 

I had his problem after replacing the T-Stat and water pump on my '07 MCS. After we buttoned everything up and cranked it the expansion tank boiled. Turned out some coolant or lube of some sort got on the friction pulley that drives the water pump. The drive wheel was slipping and not turning the water pump consistently. Sprayed the friction surfaces with brake cleaner a couple times and never had another problem. Took a little while to find the culprit. We even put dye in the system to try and find a leak.

 

Did your cooling system work as it should other than the coolant expanding too much in the expansion tank?
I have changed the cap, the expansion tank and still have not solved this problem. I can completely manage this by running the AC to maximum cold before shutting the car off, takes less than a minute. If I do not do this I get the problem

 

Hello, I was wondering if anyone could help me. I have a 2007 mini Cooper base r56 that I recently purchased. I would drive it and the coolant would start boiling or bubbling in the coolant tank and spraying from the expansion tank cap and it would start overheating. I replaced the thermostat with the housing, the head gasket, and the coolant line going to the thermostat from the water pump, and it still does the exact same thing and it's getting worse. If someone could help that'll be much appreciated. Thanks

 

I never got out of the shop. We were burping the coolant system when the problem started. Running the A/C on Max might have helped, but we were trying to solve the problem before putting it on the road. Once we cleaned the drive wheel for the water pump and it actually started spinning the pump, the problem went away.

 

Have you tried the inexpensive fixes like getting a new expansion tank cap and tank?

 

No, I will though thanks for the suggestion

 

Well I pressure tested the cooling system and found no leaks. I was not able to test the new cap, as the test unit did not have an adapter to fit. I bought the Uro parts cap, any opinions on that brand cap?

 

I don't have much love for URO parts seince the first time I used their friction pulley the plastic housing melted down and caused much more problems than just paying for a new friction wheel itself.

 

I just bought an uro cap and it fixed the problem for now at least

 

Quick recap: thermostat had a small leak, car otherwise fine, first OEM thermostat replacement leaked like a sieve and upon replacing with one that worked I acquired a boiling over expansion tank after the car was full operating temperature (especially after turning off the motor), new pressure Uro Parts cap didn't fix it, new OEM expansion tank didn't fix it, pressure test found zero leaks.
Problem solved by buying another new OEM expansion tank cap.
Moral: new parts are not reliable, be they or cheap Uro brand. Both parts that actually failed had faulty treatments in the first attempt to solve the problem.

 

Actually have (2) 2010s.

Anyway.....one had crazy boiling going on in the tank....coolant level up / down plus overflowing at times...

Scan tool showed pretty much normal temps.....no warning messages.

Tank had very subtle crack at the base of the threads. Easy not to notice.

You should consider the entire tank to be the 'cap'. Any cracks or leaks will cause boiling.

$125 for a new tank and cap at local Mini dealer and all is back to normal.

Hairline crack at base of neck.

 

I bought my 08 R56 base back in late January 2020 with 127,000 miles. It drove somewhat okay for the first 3,000 miles, with one instance where I had to replace the Crank Case Vent Hose that broke. Other than that one hose this manual nonturbo has been a lot of fun.

recently while driving in the city. The coolant started to boil over and pour out, everyone on the street yelled it out to me thankfully, Before I blew the engine completely.

I towed the car home. I already suspected the fan wasn’t engaging since I’d never heard it.

I checked the relay, with a 9V battery, to energize the relay, and checked for continuity with my multifunction reader. Everything is good with the relay and with the fuse.

I checked the pin that pushes the waterpump pulley into place, so that it can run off the friction of the Serpentine belt, it looks fine.

I traced the fan line to the power source figure i would try to jump it. found that it wasn't even connected. Reconnected it and started the car. The car reached 205F and the coolant started to boil over again.

I thought it may have something to do with the ECT sensor wiring. But when i unplugged the ECT sensor with the car on the the fan goes into safe mode and turns on. And I was getting a reading of the temp on my OBD reader. I even pulled the ECT sensor. Dipped it into boiling water and got a reading to my multifuntion reader.

i decided the problem might be the coolant itself. since it was green, it thought maybe the unknown coolant had a lower boiling point then the BMW stuff. While doing the coolant flush i also replace the T-housing, in case that was the problem. Replaced the housing and coolant to the right bluegreen BMW brand. Flushed the radiator by plugging the garden hose and running lots of water through it. Flushed the heater core the same way. Put all the new stuff into the car. Plugged everything back on: electrical and hoses. Bled the system; car on cap off bleeder valve open used approx 1Gal.

Took for a drive kept watch on temp reached 205F, And still rising, no fan. Tried to bleed system and top-off with coolant. But I could tell the coolant was starting to boil over again. Nothing has changed.

my last thought is the plug that goes into the T-Stat housing in the back, but I don’t know what to do. I am still boiling and the reservoir is blowing up like a ballon; the fan still not engaging at 205F.

does anyone have an idea I can follow???

 

just to update my status.
I have a cooling fan not engaging problem. And I had a reservoir tank boiling over problem.

Not the relay. I checked it out. Nor is it the fuse, it is also fine.
not the ECT sensor, I am getting a reading on my OBD scan tool.
Not the wiring, as the fan does know to go into safe mode, when the ECT is unplugged.
I replaced the thermostat, and did a coolant flush. so I am pretty confident that was never the problem.
upper and lower radiator hoses are warming up, so i believe there is circulation.

I just changed the expansion tank cap. Last time i looked at it and noticed the rubber O-ring was broken. It seems that was the source of my boiling over problem.
love how a $25.00 new cap solved that part of the problem.

the fan is still not engaging, and the car is over heating. I turned on the car on, set the A/C to high ( air comes out mild), and still no fan. I believe the fan not turning on was what killed my first cap.

if anyone has any suggestions where to look, how to diagnose the fan, it would be greatly appreciated.
I am temped to order a new fan but i think that wouldn't solve this problem.

 

Have you checked the coolant temperatures reported via the coolant temperature sensor?

Do the coolant hoses get warm as the engine heats up from cold?

 

thank you for the reply.

I did watch the temp go up, on my Creader OBD scan tool. So i am confident the ECT works fine. When I unplug it, the fan spins, I have read, because it goes into “Safe Mode.” I kept the ECT sensor plugged in, spliced into the wires: fan is off, direct connect the two wires, the fan turns on. So i am pretty confident all the wires are plugged properly.

the upper and the lower hoses do get warm. I am not taking the car to the highway, so I don't have cooler air blowing on it. The fan isn't spinning, so it is not cooling down the radiator.

As I am driving, i am watching the temp rise. At one point it got as high as 215 F. Still no fan.

 

215 F is too low for the fan to come on, I've seen 220 F on mine without the fan coming on.

 

Thank you giorgos, i will try out next time I go out for a drive, getting the the temp up to 220+ F. But at the same time, when i turned on the A/C shouldn't the coolant fan have started?