My 2006 Jeep cherokee did the same thing, NOPE, no blown head gasket but air in the coolant system from a failing water pump. I drained the system, did a thermostat, and water pump, then bled the line (left the bleeder valve off and radiator cap off with heat on), then slowly, slowly, slowly filled the bleeder valve until it was full, and all bubbles came out. Then I knew it was bled (took about 30 minutes of slowly adding and letting air release), and put on cap and bleeder valve, then it pressurized up, and no more overheating, no more overflow boiling.
Yes this feels like a head gasket, as they resemble the same thing, but if you shut down the car, let it sit for 5 minutes or so, and its still boiling, it aint the head gasket. The car isnt running, there is no pressure from the engine block, so no exhaust is exiting the engine and entering the overflow, instead the overflow is physically boiling from the issues preventing the car from cooling down. In my instance, it was the water pump and air in the system. I didnt change or touch the engine block (head or gasket) and after fixing the overheating issues, it no longer bubbles the overflow or boils the coolant because there is no head gasket leak, nor anymore issues with overheating!
Keep it in mind, its not always what it seems.
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