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If you are sure the Thermostat is correct, installed correctly & there is no collapsed Radiator hoses:
You need to have a Reputable Radiator Shop perform a Gas & Pressure Test to see if the System has a Internal Leak.Head Gasket leak near a Cylinder can cause high pressure in the Cooling System & stop the flow of coolant or back it up out the Expansion Tank.Thermostat is set to fully open at 206 degrees
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