The overflow tank is still bubbling with hot coolant after making sure that the radiator was okay and the thermostat is repaired, but it continues to bubble. What is the problem?

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    Hello and welcome to mechanics.stackexchange please try to elaborate more on your question in order to be more specific. – kokobill Jan 17 '19 at 8:32
  • Is the system building up pressure? Is the pressure cap sealing? – Solar Mike Jan 17 '19 at 8:32
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    That is the cylinder head gasket is bad or loosened it let's the compressed mixture into the water circuit – Chebhou Jan 17 '19 at 8:55

This could be a sign of a blown head gasket. This would push combustion gases into the coolant. Find a mechanic who owns a co2-testing device and have your coolant tested for traces of co2.

Example of co2 tester

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