New to Forum. Hello and Need advice/suggestions to save me from self admission to an institution. Recently acquired used (89k miles) 2013 Cman S All4. Drove it a bit for about 2 weeks with no coolant problems. Have been bringing all the maintenance up to par including a coolant drain/flush fill. That's when it started. Thought it was an air pocket that the venting failed to address. After multiple re drains and fill including doing a new vacuum fill it still overheats/boils out. Proceeded to replace thermo housing with prepacked sensor all from Mahle. (included the water pipe replacement which is a major undertaking imo) all finally installed perfectly. Vacuum fill and drain 2 more times each with same overheating and boil out. Noted that at 212F+ the lower radiator pipe is as cool as a cucumber. Thermo must not be opening and engine heat boiling up through upper radiator hose and then back into the expansion tank (no cracks). Cabin heating circuit working very well and midly cooling the engine as ECT slowly drops when on.(hence water pump must be good). I have schematics obtained from Mitchell1 and have completely verified good wiring condition from the DME to the temp sensor and the thermo heater. I got good wires and contact!. WHAT ELSE COULD be causing this. I also observed the fan never came on even with ECT reading 220 F via scan tool. (yes AC will turn on the fan) (BTW also tested the coolant for a head gasket leak : negative) NO OBD2 or BMW codes.
update:it is fixed. As all checked out electronically the problem was a failed pressure cap. I did failure analysis and found the cracked oring in the cap...what an idiot I was.