bear with me, this is my second post so if its not in the right place, please just redirect me haha

ive had my 07 cooper convertible sidewalk (i dunno if that matters) since 2007 and it has close to 70,000 miles on it (we do a lot of road trips), and i have never had this issue, but about a month ago, this started: a fan kicks on and stays on after i turn the car off. it turns off in 2-3 minutes. we noticed coolant levels were low, so we put new coolant in it, and for a week or so, this issue stopped, but its slowly picked up again. and now there have been two occasions where we stop driving, and were simply idling for a few minutes (5 or 10 min.), and the fan will kick on (after not having come on the entire time we were driving, its a very loud fan and definitely audible even with windows up driving), and the car will overheat. ALL while idling only. the car has never overheated while being driven. its a very fast spike in temperature, as soon as the fan comes on, the temp rises, and very quickly. if we turn the car off and back on, the temp is normal, and the fan is off.

ive heard ideas ranging from coolant levels (which we check regularly), to thermostat, to bad censors, to blown fuses. has ANYONE had a problem like this? and if so what can i possibly do to fix it? also the car is no longer under warranty.

 

Air in the system.

 

It likely got so low, when you added coolant, air is stuck in the radiator/hoses. It just needs to be bleed. One tip... Look for the leak first...the two most common places on gen1 mini's are the thermstst, and the tank itself...it leaks around the seam.