Hello and welcome, Super mechanic here. Thank you for trusting us with your vehicle concerns, I will do my best to help you with your question.
That could very well be the thermostat, there was a service bulletin on some of them for a revised part. Also, when you turn the heater on, the heater core will act like a small radiator, so you will want to try to feel around the radiator's core when it is warmed up to see if there are any cool spots. That would indicate a clog.
Here is what the bulletin states **** BULLETIN *******
The first thing that you want to do is check the cooling system for any exhaust gases. There is a tester for that, that you can gt from Autozone for about $20. It comes with instructions, but there is a blue fluid that is used in it, if the fluid turns yellow, that is exhaust gases.
Block tester available as:
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OEM/Block Tester AUTOZONE
Part Number: 27145
If there are any exhaust gases in the cooling system, that will cause more pressure than desired and will allow the engine to burn coolant and/or heat the coolant up too high. Most times not filling the crankcase with coolant either.
If there are, then there would be an internal engine problem, like a head gasket or head.
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