I have searched and read all the threads I could and could not find an snswer, so here goes.
Car has 40,000 miles.
No coolant leaks.
I started by replacing the obvious:
coolant temp sensor
both low and high speed relays
thermostat housing
all new European coolant.
Engine reaches 207 degrees and fills and overflows tank.
FAN DOES NOT COME ON. NO CODES NO LIGHTS.
Fan comes on with ac on, can manually make fan come on with diagnostic tool, and fan will run continually with temp sensor unplugged.
My thoughts would be tank cap and/or fan resistor?
Car puts out good cab heat and cooling ac also
Even after all the replacement parts the exact problem exists, I'm a little baffled here.
Thanks!

 

Perhaps you have an air bubble and the temp sensor is not reading right and able to control the fans and rest of the fan cooling system that you've tested and proven to be OK? So, if you unplug the sensor and the fans run continuously have you then tested to see if you get that overheat/boil over condition?

 

Maybe something to do with the fan resistor(s)? (like you mentioned)

Just a thread I saw once upon a time. Doesnt look like a definitive answer though. https://www.northamericanmotoring.co...ng-2009-a.html

 

No overheating with the sensor unplugged and the fan running. At 70 mph the engine diagnostic is reading 211 degrees, with no boil over.

 

That temp sensor in the elbow under the intake tube. Did you replace that with an OEM bmw one or a cheaper one?
Mine was a bad cheap sensor.. I didn't spend the $50 for a bmw one and got a $12 knockoff on amazon. - Caused a lot of the same issues.

When you replaced the thermostat housing, did you get the correct adapter harness? its based off Month and year the car was made... Mine was the incorrect one there too.. so it was using the temp sensor in the elbow (which was the bad knockoff one) instead of in the thermostat housing.
There should be NO cable going to that sensor in the metal elbow. its unused once you have the correct harness in place for the thermostat.

Edit: just saw this was a 2014, my apologies.. you wont have that sensor in the metal elbow under the intake tube.. (I think they got rid of that in 2013)
My thoughts go to incorrect harness to the new thermostat housing/ bad temp sensor.

 

Everything is oem replacement parts.
Same results every time.