I have replaced every part of the cooling system including the pump. Air pockets form in the cooling system, causing thermostat to delay opening. Compression & head/gasket check out okay. There is a technical service bulletin on the exact problem, posted through Mitchell in 2005. Dealer says this TSB doesn't mean anything. My mechanic says thermostat casing is faultily designed, traps and builds air, causing problem. TSB says "REPLACE NO PARTS" and call drivetrain hotline. I have parked the car, w/ 47K miles on it and had to buy a Honda to drive while I figure this out.

Anybody who knows how to track the TSB for diagnosis and prescription please contact me. Mini Cooper is of no help, even denying that the TSB exists.

 

The thermostat housing and gasket are common sources of this problem. If you have not yet replaced them, I would start there - actually a relatively easy and inexpensive fix.

 

Yup...if the system was not bleed properly, or there is a slow leak, like a thermostat (common on 05's) or the coolant tank (very common, they leak around the seam..should look milky yellow, not clear in the most recently revised part), the system will run low on coolant, suck air in...and then you add some coolant....and the car overheats...due to air beaing trapped in the cooling system.
Also a common problems on most mini's, including the 05, is the cooling fan....the low speed part fails...resuling in it running only on high speed, and it turns on only just in time to prevent the car from overheating....that and when the ac is on...you should hear the car's fan turn on, and stay on for a bit when it is warm with the ac off...pretty quietly...if with the ac off, it turns on and off every minute or so, and it is very loud, then you have confermation that the low speed resistor on the fan has failed....aftermarket fans are avalable for $85-100, and fit perfect, oem is $300 ish.
If you don't ever hear the fan, it is possible the wireing harness was never upgraded on your car....some early 05's were delivered so that the ps fan, and the rad cooling fan are on the same fan...the ps fan gets jammed with grit...gets stuck, and blows the fuse rendereding the radiator fan inoperative.
An 05 should have been rewired after delivery, or was built with the upgraded harness...mine was updated a few months after delivery.
Good luck.... And with such a seemingly simple problem...time to find a new shop...any mini shop should fix it easy!!
And remember...use high octane fuel only...using less than 90 can cause cracks in the head....blowing headgaskets, causing similar problems to yours!! But before you worry about that, there are other things to check out!!

 

PS...welcome to NAM!!