My question is what causes water to bubble in my overflow tank?
A blown head gasket that is blown between a coolant passage and the combustion chamber can cause combustion gases to be pushed into the cooling system. This pressure will show up as air bubbles and overheating of the engine. It may even push out coolant from the system. A mechanic like one from YourMechanic can test the system for these gases using a tool that detects the gases in the cooling system from the combustion process. A pressure test can also put pressure in the cooling system and see if it can hold pressure. Once the tests are done the mechanic can then determine the head gasket is leaking.
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