BMW Coolant – Radiator Expansion Tank Cracking, Splitting, Leaking – Exploding
This answer is applicable for many BMWs in addition to the one listed below.
car year: 2007
car model: X5 4.8i
My x5 overheated and all the coolant shot out of the little hole at the top of the expansion tank. The dealer replaced a faulty fan relay which was diagnosed as the culprit. A few days later, with no signs of overheating, the the expansion tank exploded (splitting horizontally, three inches from the top) spilling the coolant. What do you think is causing this? Do you think it has to do with recall # 11V-521?
We would not think that your current cooling system problems are related to the Turbocharger Auxiliary Cooling Pump recall issue.
Unfortunately, the problem with the coolant system expansion tanks cracking and/or splitting is a somewhat common problem on many BMW models. While this issue does apply to practically all BMW models with plastic expansion tanks, this particular tank model applies to all X5 and X6 models from 2007 to current. You did not mention if the dealer replaced the expansion tank when taking care of the fan relay. It sounds like they did not.
Since you noted that the engine temp was normal, we will assume that this is indeed an example of the common expansion tank failure of cracking or splitting under the normal system pressure. The cure is to replace the expansion tank and refill the coolant.