Hello, after my last trip, my Mini Cooper (2002, r50) had bubbling in the coolant expansion tank. In addition to that, I noticed the coolant level slowly goes down (I’ve refilled it after buying it a few months ago, and had to re-refill it recently as the level was again below the minimum).

Now I found quite a lot of topics of bubbling in the coolant expansion tank, and it can be from pretty much anything in the coolant system, which is why I ask your help on how to know which component is bad using a systemic method, rather than poke around blindly. At this point, I’m not even sure if it’s gas bubbles or boiling bubbles, so I’ll explore both.

Hopefully it can be useful to fellow Mini owners who have this issue.

A few methods require knowing the coolant temperature, which can be seen using the secret menu or using an OBD2 tool.

Note that I focused on a r50, which has a non-pressurized coolant expansion tank and a thermostat cap under pressure. R53s have a different system with well documented weak pressurised expansion tank.

• First is the dreaded head gasket. Exhaust gas leaking in coolant could cause bubbles. That one is easy: use a cheap leak tester that you plug on the tank, and if the testing liquid changes color, bubbles are exhaust gas. There can be other symptoms of a failed head gasket (white smoke at the tail pipe, creamy oil), though I've not seen any other in my case.
• Improperly bled system (either due to last coolant change or if coolant got too low?). Easy again: open the bleed screws, see if there is air.
  
• Broken fuse for a cooling component: unplug fuses, visually check if they are broken. Easy again
  
• Defective fan: the low-speed setting should turn on at 105°C coolant temp and high-speed setting should turn on at 112°C coolant temp. Rev the engine after warming it up to reach those temperatures. Not too hard apart from needing a coolant temperature reading.
  
• Thermostat doesn’t work properly: at 91°C coolant temp, you should hear a light click and feel the upper radiator hose starting to get warm. Any other testing method?
  
• Thermostat cap leaks pressure: apart from changing it outright, is there a cheap way to test it? Dedicated tools seem to be too expensive for the DIY mechanic.
  
• Water pump doesn’t work properly: I found a suggestion to pinch a hose and feel if there is flow when releasing it, but is there a better way?
  
• Clogged radiator, mainly due to lack of coolant changes: if the upper hose is hot, but the lower one is cold, the radiator might be cold. Is there a better way to test that?
  
• Damaged radiator, due to debris impact: visually inspect fins. I think it would cause engine overheating issues rather than coolant boiling? Otherwise is there a way to test actual performance?
  

Is there any other part I forgot that could lead to bubbles/boiling in the coolant expansion tank?

Thanks!

 

First, how many miles on the car, and when did this first happen, like during / right after the recent cold snap? Might help others weigh in...

Would start with low hanging fruit:
>Did you replace the cap with a new one when you replaced the tank? Check / re-check the clamps for tightness
>Piece of cardboard under the car after it's been warmed up, driven a bit, and parked to see if you have a slow gravity leak somewhere.
>Dye in the coolant system, UV light + glasses, look before and after car warms up. If possible put the car up on jacks and pull the skid plate to have a good look around / underneath, especially those 857 or so coolant hoses between the engine and firewall.
>Leakdown test

Frequent related issues on these cars:

Cap as noted above
Thermostat housing / gasket - replace with a metal one from Uro. [Edit: sorry, my bust. These only exist for S model R52 and R53. Unobtanium for R50. D'OH!]
Upper radiator connector / bleeder - made of crappy recycled plastic. Also replace with metal Uro set with the brass screw. Rock Auto and Outmotoring have them. (Be sure to install the thermostat with the little air hole in the 12 O'Clock position to ensure complete system bleeding.)
Other plastic hose connectors in the engine, one "X" and one "T" behind the engine, kind of hard to reach
Radiator, heater core, water pump like any car with time and miles
Oil cooler (R52/R53 Cooper S with manual gearbox only)

If it does turn out to be a coolant system leak follow the Pelican Parts tutorial on system flush / bleed, very good. These cars are notoriously hard to bleed. Front end elevation is key.

https://www.pelicanparts.com/techart...ant_Change.htm

Would replace the fan resistor regardless - just assume it's already toast.

 

Thanks for the answer!
The car has 130k miles on the odometer, and there was no cold snap where I live. Temperatures dropped a bit to -5°C / 23F a few weeks ago, but it was a nice 5°C/10°C (40F/50F) the last few weeks.

I did not replace the expansion tank. As far as I understood, the expansion tank is only an issue on the turbocharged r53, but is not an issue on the naturally aspired r50.

Indeed I forgot to list that one. My car is nearly always parked on cardboard and I check it everytime I drive away. I did not notice any leak.

Nice idea. I'll add that to my list of troubleshooting.

Is that useful to do if I can confirm there isn't exhaust gas in the coolant tank and there is no other head gasket failure symptoms?

Thanks a lot for that list! I'll put all of that on my list of things to inspect/do if I can import those parts or find them locally (I'm living in EU).

 

The bubbles can be caused by a head gasket leak or coolant boiling. If the bubbles stop when the engine is shut off, well, that's a head gasket (or a cracked cylinder head) leak.

If the bubbles only happen when the engine is fully up to temperature that is likely do to a lack of sufficient pressure and this is a leak.

There is a test that can be done to test for the presence of combustion byproducts in coolant. Don't think it is available in test strip form though.

Coolant can boil if the engine is shut off hot -- with the radiator fans running and the A/C off -- if there is a pressure leak.

That the coolant level goes down suggests (rather screams it...) there is a leak.

My SOP is to recommend one do a hot pressure test of the cooling system. Top up the coolant level with distilled water. Do not overfill the system but the fuller the better. You want as much of the cooling system submerged under coolant as possible. (Leaking coolant is easier to spot than leaking water vapor.)

Start engine. Turn off A/C. A/C on has the radiator fan on and this tends to keep the coolant temperature down which is not want is wanted in this hot pressure test.

Drive the car around town to get the engine fully up to temperature. To the point the radiator fan is coming on. Back home on your driveway raise RPMs to ~1K and hold until the radiator fan comes on. Shut off the engine. The hot engine will raise coolant temperature and this raises pressure. If there is a leak -- and it is covered by coolant -- you'll see the leak.

Be careful. While you have the hood open to expose the engine and have a flash light and ideally a mirror on a stick to look at all the possible leak areas hot coolant can spray or gush out at any time. I was testing one of my cars and under these hot test conditions a bad coolant tank gushed hot coolant to the ground. About a gallon of it. A split along the mold seam had developed a crack.

You can also make an aluminum foil "tent" to place over the coolant tank cap or any cap that is part of the cooling system. If the cap is venting pressure it will vent water vapor (the anti-freeze remains behind) and this will condense under the foil tent.

 

I only noticed the bubbles after the engine is shut off (they might have been there before, but I didn't notice the sound before). That's good to know, though I'll still test for head gasket failure in order to be thorough.

Since then, I only tried to start the car once for a few minutes (lack of time, it's not a daily driver), and there was no bubble. Right now I go on the assumption that it only happens when the engine is up to temp. On the other hand, there was no visible leak on the floor.

It is available for cheap. I got a kit for 50 tests for 20€ on Amazon (there are cheaper ones on Ebay for less uses). That's actually the first item on my list, and the first thing I'll test for when it's bubbling again.

Interestingly, when I noticed the bubbles, I also noticed the fan was not running, which did strike me as odd. Too early to draw a conclusion, but I'll definitely be testing if the fan runs properly, which is the fourth step in my list. Considering Daftlad mention of fan resistor going bad and that I also saw that issue several times since then, I have a high suspicion something's going on there.

Yup, I want to find out where that coolant is going, since it's not on the floor, it has to be going somewhere. Unless I spilled it last time I moved the expansion tank recently (for access to the starter), but I really doubt I spilled that much.

Thanks, I'll add that to my list of tests.

Many thanks! I was looking for a way to test the thermostat cap without specialized tools. I will use that method.

 

@Daftlad , do you have a link to the metal thermostat housing? I've seen the metal double connector/bleeder that sits back in the engine bay, and I was considering tossing one in the cart for my next parts order. However, I've never found a metal thermostat housing for my car. I would love to have one, too. My plastic one cracked back when my car was under warranty, and I've just been waiting for it to happen again. So far so good though. I would also love a replacement for the upper bleeder, too, though I suppose that might be easy to just match up with a simple 90° from another car.

Speaking of cracked thermostat housings, the dealer techs said they had a heck of a time figuring out where the leak was. Apparently it was fairly minor and only happened at operating temp, IIRC. In fact, I don't think I had coolant on the ground from it, but it has been a very long time.

By the way, I've never found the R50 to be difficult to bleed, but its cooling system is fairly different from the R53. Maybe that's why?

 

The coolant may not hit the floor. I smelled anti-freeze on a hot day. Took my car (oh not a MINI) and tech lifted the car and almost immediately it was clear the water pump seal was leaking. But it only leaked when the engine was fully up to temperature. What would happen is heat and the resulting pressure would force some coolant out past the water pump seal. But being the coolant was hot and the engine was hot the water in the coolant would boil and turn in to vapor. The anti-freeze remained behind to form a nasty looking crust of anti-freeze crystals.

Think I have a pic:


Had I not "caught" this in time coolant would have hit the floor eventually.

Another source of coolant loss with no signs of any coolant leak is via a cap on a coolant reservoir tank that under similar conditions as the water pump seal was leaking will let water vapor out. No anti-freeze is in the water. It is essentially "distilled" water. But the amount of water that can be lost can be quite large. I had one car that a low coolant warning light came on. I checked and sure enough the level was quite low. Took almost a gallon of distilled water to bring the level up.

Checked cap and it was not loose, or put on crooked, and showed no signs of any issues.

Checked around as best I could for any leak sign. Found none. Put the car into regular service (60 miles/day work commute). In under 30 days the warning light was back on. But when I opened the rear trunk lid -- car was a mid-engine layout -- I noticed some drops of water on the underside of the lid the portion that was over the coolant cap. A light went off and that's when I decided to test for a leaking cap with the tent of foil.

And sure enough water appeared. A new cap and no more low coolant.

 

Guys my bust on this one. R53 has the aluminum housing from URO. Still unobtanium for the R50. Sounds like a pack up kit item.

Regrets
Daftlad

 

Dangit. I had my hopes up and everything.

 

Small update after testing a few things (not all the list because I didn't have time nor the right place):

• The system is bled correctly, there was no air when opening the bleed valves.
• The coolant was right at the top in the thermostat. I could not notice any damage on the thermostat cap (though it can leak without visible damage), nor any wetness or steam around it with the engine running or after a long drive.
• All small fuses are good. I do not yet know how to remove and check the big ones, I'll have to learn that. It's unlikely there is an issue there.
• I connected on OBD scanner and did 2 40min drive. When driving, the coolant slowly goes up to 96°C/99°C, then quickly back down to 88°C/90°C (I supposed that's when the thermostat open), then slowly back up to 96°C/99°C and repeat. This seems to indicate the water pump, radiator and thermostat work well enough. On the other hand, is that behavior normal, or could the sudden temperature drop be indicative of a sticky thermostat?
• The first time I stopped, the coolant shot up to 99°C and stayed there, with no bubbling/boiling.
• The second time I stopped, the coolant shot up to 103°C and stayed there as well, with no bubbling/boiling either.

Here is a chart of coolant temperature during the second drive, I'd appreciate opinions on if it looks normal:

Coolant temperature over time


With the information I have and considering it's a common issue on those models, so far my hypothesis is that the low-speed fan resistor is shot, and shutting down the car at just the right engine temperature (between 105°C and 112°C) leaves the coolant system without proper cooling, which leads to coolant boiling after stopping the engine. When boiling, some of the coolant escape (because the r50 expansion tank is not pressurized and actually has an opening).

While looking up how to disassemble the fan, I came up on this video guide that explains exactly those symptoms as well:
My next step is, whenever I'm at a place I can rev my engine freely while stationnary (not possible in my underground parking lot, and I'd rather not do that in the street), to bring the temp up to 105°C to check if the low-speed fan turns on. I'm pretty sure it doesn't already, because I only heard the fan ever turn on when stopping the engine 2 times in the 4 months I've owned it, and considering how noisy it was, I'm pretty sure it was the high setting.

On the other hand, is there a way to test the low-speed fan by jumping wires without disassembling the fan / the whole front-end?
Edit : found it, it's different for 2002/2003 and later models, here is a video guide for a 2002 model:

 

You'd think by now they'd offer one. Replacing the crappy plastic on these cars with metal should have bottomless aftermarket potential. My personal favorite: the clutch slave cylinder. 67K miles on the car, just installed its third one.

 

I know, right? They offer one of the other R50 specific connectors, so I was really hoping they had finally started producing the thermostat housing.

 

I dug out one of my old log files that has coolant temp in it. Unfortunately it isn't very long, but it does have a cold start.

 

@deepgrey Thanks! That was what I looking for. Your log shows the same open/close behavior (rather than a partial opening that would lead to a smoother coolant temperature graph as some people told me should happen). It looks like your thermostat opens at a slightly lower temperature (about 95°C/96°C), but that might just be what happens when everything is getting up to temperature.

I conclude that there is no cause for concern on the thermostat part right now, though I will keep it under supervision long term.

 

A little late to the party but the up/down saw tooth coolant temperature waveform is normal.

A pic of a graph from my 2020 M-B cargo van. But I have data logged a number of cars over the years and the coolant behavior is very typical.




Might add driving with the A/C on smooths that out a bit as the radiator fan runs all the time and this works to keep the coolant temperature more stable.

 

You're welcome. Apologies for the scaling; the temperature starts to decrease around 93 to 94°C, which should mean that the thermostat opens a bit lower, likely right around the correct 195°F mark.

By the way, the end of the log (starting around 775 sec) is with the car stopped and idling.

 

Last update: I confirmed my suspicion: the resistor from the low-speed fan is dead.
Since I could not rev my engine to bring it to the low-speed fan temperature, I used an external 12V power source (I could have used the battery, but I had one lying around) to test the fan itself:

• when putting 12V on the high-speed fan relay connector (small 2-pin connector at the top right of the radiator), the high-speed fan turns on correctly.
• when putting 12V on the low-speed fan connector (big 3-pin connector right beside the previous one), the low-speed fan doesn't turn on (it's already past the relay in the fusebox, so it cannot be a relay). The positive side can be identified by the fact that it's the smaller of the red/blue wire (and it's right beside the negative). The bigger fan is the power source for the high-speed fan.

Since the high-speed fan turns on but not the low-speed fan, according to electrical diagrams, it can only be the resistor that's bad. And since this is a common issue on those Minis, no need to look further.

With that said, I'm faced with 2 options:

• buy only the resistance, which is NOT an official part on that Mini (it's only a replaceable part for 2003+ Minis). According to this thread, a Dorman 902-706 is a compatible part. On the other hand, Amazon has a few scathing reviews of it, it's more work, and you only replace the resistor, meaning the relay or the fan could fail later on and you'd have to redo the repair. Price is around 20$/30$ (or a whopping 55€ if you try on Amazon europe).
• buy the whole assembly fan (be careful about choosing the right one, most sites will show both pre-2003 and post-2003 on a search by model/VIN and it will only be a small note). While an OE/OEM one is going to be expensive, I found a compatible one locally for as low as 80€ (unknown brand "THERMOTEC").

With all that considered, I'm going to personally choose a whole assembly.

Thanks! Yup that's a bit lower than what I saw, but I'm not overly concerned about that since it properly opens well below the low-speed fan threshold.

 

Another car of mine one radiator fan only ran at high speed. Was the ballast resistor in the wiring harness that failed. I just had the harness replaced. Was not that much money.

Might mention the tech told me the ballast resistor can fail prematurely if for some reason some one has been at the car and the radiator fan motor electrical wiring harness and not put the wiring harness back the way it belongs. The ballast resistor runs hot -- normal -- and the factory arranges to put it where it receives good air flow but where it is not subjected to water splash and is secured so it doesn't flop around.

In the case of my car the wiring harness was right where it was supposed to be. The resistor just failed. I can't remember the miles now but think it was after 2006 (the car was bought new in 2002) so based on my memory the car probably had 100K miles. The replacement (in the new harness) lasted way longer than the original one and was still working good when I sold the car in January 2018 with 317K miles.