UPDATE

This x5 35i N55 doesn' have the B604 coming off the radiator neck...so odd.

Cap arrived, no change, still boils over.

Autozone had a coolant temp sensor, changed that thinking the sensor was reporting incorrectly, still no change. I think the coolant tank may be cracked and leaks under presure only...there are zero leaks untill it hits 102C+

Video of expansion tankHERE - you see bubbling from the evacuation hole, but it isn't coming from the cap.

ordering expansion tank now.

I guess we should have asked, "Where is the coolant coming out of?"


What you describe is classic coolant tank failure. The overflow is only used wehn the car is parked- then coolant is lost..eventually the coolant is low and people wonder 'how, its fine when I drive it around' Its that hot/off/cool cycle where the cracked tank lets the overflow leak


Lets hope....

Its funny... the leak is coming out of the cap and down the evacuation hole...when I trigger the ac button fan come one cools it and the cooland stops coming out of the cap, but a little bit is coming out the evac hole...I'll take a sawsall and figure out where it is coming from once I change it.

SOLVED - It was the expansion tank... the top section meets the bottom at the evacuation hole. Video HERE

Fan triggers on at 109C and no leaks.

Video Here https://www.dropbox.com/s/qkkrwqygjb...42050.mp4?dl=0

Damn that is a sneaky location for a leak.

Nicely done. Especially the “29 is 2 bar” part

;)