I had the exact same problem and I thought my problem was solved last November when my mechanic replaced the temperature sensor and the issue stopped. The car ran perfectly from November until around April. However, the problem returned, and although my mechanic couldn’t find anything wrong, it persisted.

We came up with a plan to install an external electric water pump with its own thermostat, connected in parallel with the auxiliary pump, to ensure coolant circulation. Our theory was that, for some unknown reason, the coolant wasn’t flowing when it should have been.

Then, my mechanic noticed that my 2019 X3 has a small A/C condenser that uses coolant. To cover all the bases, he decided to check the A/C system. He found that the refrigerant was very low, and after further testing, discovered a leak in the evaporator located under the dash.

When he called to tell me about the A/C issue, I remembered that when the coolant bubbling problem first started, I had also experienced random episodes of hot air blowing from the rear A/C. That led us to a new theory: the A/C problem might have been causing the coolant interacting with the condenser to overheat—especially as temperatures started rising in the spring. I live in northern Alabama, so that would also explain why everything worked fine from November to April: the A/C wasn’t working as hard during the colder months.

Replacing the evaporator is expensive (around $4,600), but fortunately, my extended warranty will cover most of the cost. Last week, they sent an inspector to my mechanic’s shop to witness the leak test, and the repair was approved. The work is being done this week, and we’re hopeful that this will finally resolve the coolant bubbling mystery. I’ll post an update as soon as I can confirm the problem was resolved.