I recently purchased a 2002 R50. After driving, or sitting idling for extended periods, the coolant in the expansion tank boils and raises to the point of overflowing. About 30 seconds later the fan kicks on, which brings the temperature down fairly quickly. When it does, the coolant level in the expansion tank will drop to a "normal" level; between max and min. This cycle continues while the engine is running. Note that the temperature gauge on the dash shows it right in the middle.

After it cools down (engine shut off) the coolant in the expansion tank gets pulled back into the engine and leaves the expansion tank empty. I've filled it back up to the "Minimum" line but the same thing (boils over) happens next time its run.

So far I have focused my attention on the fan, but I'm 90% convinced it is working properly. The fan comes on at 105C and goes off at 101C (using the temperature displayed from the "Test 7.0" from the Mini's computer.) The only thing that concerns me is that the fan does not come on when the a/c is running...I've read it should?

Can anyone give me some ideas as to what may be going on? ...what I could check next?

 

Your Low speed fan is not working. Just replace it..this issue is very very well documented.

Coolant boiling over is one of the symptoms that you can see.

 

The low speed fan is supposed to come on at 105C/221F and go off at 101C/214F, right? ...that's exactly what happens. But it starts to boil at around 103C; just before the fan kicks on. Once the fan comes on it drops the temperature fairly quickly. It hasn't become hot enough (112C/234F) for the high speed to kick on.

 

Lowe speed fan comes on at 211...high speed triggers at 221...hence your problem....

 

I had exactly the same symptoms. I found that the system had quite a bit of air in it, and after bleeding the system properly and replacing the coolant it was all good. Until I discovered 3 minor leaks. I ended up replacing the thermostat housing, radiator, and went to silicon hoses. As it turned out, the leaks were very minor (just "seeps" really) and didn't leave coolant behind when the car was parked. They were enough to suck air into the system and lose a couple of cups of coolant over the course of a few days. Now the coolant tank balances perfectly between full and cold marks and no more overheating at the drive-through or at long lights in traffic. All components were about $220.00

 

Have you TESTED the coolant?
Weak coolant, aka, mostly water boils at a lower temp...
And with a cast iron block...allows corrosion...
Many folks ignore the coolant, but these cars came with a 3 year flush coolant, NOT the long life..
Newer BMW blue coolant IS long life, but often folks never flush...and just add water..
End result, corrosion, blown headgaskers, and blocked heater cores...

 

A new $4 radiator cap stopped the boiling over problem. ...but now I'm still unsure if it is "fixed" or not. Kahnfucious, you say the high speed fan comes on 221F (105C)? Everything else I read says that' is the temperature to trigger the low speed fan. When it kick on, its hard to tell if its high or low speed just by the sound...but I'm pretty sure its low.

With the fan out of the car, I tested the fan with a couple jumpers off of a 12v power source and both the low and high speeds work.

Post #17 in the thread below is where I'm getting my information on the temperatures. https://www.northamericanmotoring.co...-problems.html

 

ZippyNH, coolant tested good, but I plan on flushing it anyway. Its a new to me car, so no idea how old it is. It has the green fluid, so assume its not original, but still unsure how old it is.

 

Low speed fan is pretty quiet...barely noticeable...
Hi is basicly never heard in 99% of cases....but if it runs, sounds like a jet engine or helicopter... YOU KNOW it is running from the sound..
I would advise you bleeding a cooper is a pain...check for the bleed Locations...think TWO on a cooper...one kinda hidden by the thermostat...
Some shops use a vacume bleeding system on a cooper, but just know that it might take 4 or 5 tries to get all the air out....bleed, rev, bleed drive a bit, etc... Driving with a bubble WILL result in overheating...
Many debates on the "right" coolant, but mini/BMW blue is a HOAT, similar to most of the big box long life coolants, just IMO stay away from DEXCOOL (OAT) coolant, not approved.