First off, everything on my 2011 NA R55 was fine, it just started getting some timing chain rattle and opted to replace it proactively with just under 100k on it. All seemed to go fairly well. I opted to replace the water pump, WP drive wheel and tensioner/transfer wheel that pinches between the crank and WP to drive the water pump (very odd configuration to me) along with new serpentine belt. Also replaced the thermostat while at it. Went through the cooling system burping process, or I believe I did correctly. New expansion tank and cap as well. From here is where I'm loosing my mind. Not consistently but almost consistently it overheats. The one area I can't let go is when I bleed the cooling system I've never had it spurt or spray as you'd expect from a hot and cycling cooling system. I'll get steam and eventually a trickle of water once the steam purges but never a flow that I'd expect with a cycling, engine running at temp, system but this is the first Mini I've ever had. My question is, are there any chances the water pump (Febi), Drive Wheel (Dayco) or Friction Wheel (Dayco) could be bad not cycling the coolant properly? I can see the water pump turning but I have no way to determine if it's turning but slipping at the same time not cycling the coolant fast enough. I'm at odds what may be at the heart of the issue as all the key elements are new and there were no heating issues prior to the timing chain work. Engine runs fine afterwards, just overheats. I even tried a second thermostat with no better results, and went back to the stock factory one I took off, all with the same results so I'm not thinking I have two bad new ones and one that was fine for over 90k miles magically went bad. Thanks for any insights before I start tearing the engine down looking for culprits or signs of new damage from the overheatings.

 

How did you go about it?

Here's what worked for me:
1. Open bleed screw
2. Fill coolant to max via expansion tank, making sure coolant flows out of the bleed screw. Close bleed screw (while coolant is flowing out) and expansion tank.
3. Run engine while monitoring coolant temperature using the "hidden menu". If coolant level in expansion tank decreases, repeat steps 1 to 3, otherwise go to 4.
4. Make sure all hoses are warm (i.e. the thermostat has "opened"). Set cabin temperature to highest setting and cabin ventilation to lowest. Drive the car until coolant temperature is above boiling (or engine ventilator has started). Carefully open bleed screw as little as possible and let open until clear coolant flows out (i.e. no bubbles).
5. Close bleed screw and monitor coolant temperature and level until the engine has had a change to operate over some time at its max operating temperature.

If the hidden menu reports ttt (letter t thrice) for coolant temperature, the temperature sensor is outside its valid range. If the coolant temperature stays cold (or rises very slowly) and then jumps abruptly to high temperatures, there is air in the thermostat housing (where the sensor is fitted). Bleed again.

How do you determine that it overheats? From the warning light coming on? If so, monitor coolant temperature via the hidden menu in order to see how the temperature increases over time. Also: Does the coolant level stay constant or rise inside the expansion tank while the engine is warming up or does it decrease?

Do you get steam out the bleed screw or the expansion tank? If the former: what happens to the coolant level in the expansion tank while the bleed screw is open?

Do all the hoses get warm/hot in their entire length? Provided you have not forgotten to completely re-latch the friction wheel release tab, I'd be very surprised if the water pump is slipping that much that it has such an effect on the system's cooling capabilities under regular engine load. If you do have reason to suspect it is slipping (and not making any noise), you could wipe the surfaces of the water pump and friction wheel pulleys as well as the serpentine belt and the other pulleys with brake cleaner.

If all the hoses do get warm/hot, it is unlikely that the thermostat does not open. Note that there are two kinds of factory (or BMW/MINI original aftermarket) thermostat housings. In the older version, the temperature sensor was integrated and not changeable. In the newer version, the temperature sensor can be removed separately. My N16 engined LCI built in November 2010 came with the older version from factory. When the temperature sensor failed, the newer version was installed by BMW.