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Possible Cylinder Head Gasket problem. Check for any coolant leaks coming from the cylinder head gasket. We have seen head gasket failure and a few with cracked heads causing air pockets at the thermostat as well. Check if the lower radiator hose is cool when the engine is hot. If so, the thermostat is not opening. Possible air pocket behind the thermostat preventing it from getting hot and opening. Verify Heater core return has flow and there are no external coolant leaks, which could allow air to get into the cooling system.