There is an issue with the engine cooling system and irregular operation of the radiator fan, which turns on and off inconsistently. Coolant is returning from the radiator, causing the expansion tank to overflow, and sometimes it ejects coolant outside the tank.
In the morning, when starting the car, I noticed a drop in the coolant level inside the radiator. Therefore, I leave the radiator cap open to allow the release of any trapped air while adding coolant until the radiator is completely filled.
However, after driving the car, I noticed air bubbles forming inside the expansion tank, even though the radiator cap is new and in good condition.
Question: What could be the most likely cause of these symptoms?