So long story short i just bought my first mini cooper, its a 2009 Clubman S and i purchased it with no compression on cylinder 3 for cheap. I just finished rebuilding it after replacing burnt exhaust valves, entirely new timing chain kit, water pump, coolant hose, oil filter seals, etc (all the wear items a friend told me about).

I followed the directions i found on here for filling the coolant and bleeding it via the screw on top of the new thermostat housing, didn't get that many bubbles but when fluid seeped out i closed it off. The car starts up great but gave me a p0128 code. I hooked up my obdii bluetooth adapter to my phone and the car was running around 174 degrees. After about 20 minutes of idleing it reached 235ish and the fan kicked on to cool it down. The car got all the way down to 160 and i turned it off since the cooling fan was still running. The entire thermostat housing, thermostat, and temperature sensor is new, it was filled with 50/50 blue coolant. The coolant line doesn't seem to drop or change. I'm so clueless to what's going on, does anyone have ideas?

 

The code you have is for Thermostat issue.
Just because the parts are new does not mean they cannot be faulty.
I would replace a new thermostat and see if it solves the problem.
Also, could it be a faulty coolant temp sensor? Did you replace that too? If the sensor is sending incorrect data then that explains why you are getting abnormal readings.