Is the rad fan working?

 

Verify that the cooling system is actually overheating. With the speedometer and tach not operating, this would cause me to believe that you have an electrical problem affecting the gauges which includes the temp gauge, perhaps a bad ground or electrical connector.

 

 

With the BMW system, it is critical the voltage when running is 14 to 14.5 volts.
The plastic coolant resurvior can be slip open and still look fine, but it will overheat, because it can not make pressure in the system. (these always fail with age)
Also, there is a burping threaded plug on the upper radiator hose, that can be loosened to bleed off air in the system.

 

1) low/high speed fan failure for radiator. - Confirm fan is working. Usually the .03ohm resistor (green) on the fan assembly burns out.
2) Check the cap for coolant expansion tank on firewall. Cap may be done, there is a built in spring in the cap, and often need to be replaced. Caps can be purchased separately or come with a new tank.
3) Expansion tank is split. ( odor of coolant cooking off when engine running ) check level.
4) The water pump runs off the supercharger on the drivers side ( U.S. Spec car ). If supercharger doesnt spin, neither does the mechanical water pump....is there boost ? I had situation where the internals for my R53 water pump were cooked.....not even sure how the supercharger was even making it spin.....
5) Crank Pulley failure, causing belt slippage resulting in supercharger not spining = water pump also not spinning.

Few things to check off top of my head.....

 

Expansion tank upgrades:
https://www.greeneperformance.com/co...e-coolant-tank
https://www.ecstuning.com/b-canton-r...t/80-272blkkt/

 

 

Not everything...

Yes, $180 and/or $205 for a fancy-schmancy aftermarket welded aluminum 'designer' tank like the two you linked...

or, under $33 for an OEM tank, like the one that shipped with your MINI from the factory.

 

This aluminum one is $89.50 It uses the stock cap.

 

One other thing to do is swap out the thermostat. Simple easy thing to do.

By putting in a genuine MINI one in, the new thermostat stabilized the coolant temps to the point that the variance is no more than 10-12 C from putting around to hauling butt uphill. The fan rarely comes on high now, actually, I haven't heard it all since we changed the thermostat...