have you changed the battery in the thermostat?
You may have the thermostat in backwards. The spring side of the thermostat faces into the engine.
The thermostat and reservoir will have nothing to do with this situation since you have (ATC) Automatic Temperature Control and you most likely will have a bad sensor or control head, its best to have the (ATC) unit scanned for all issues........
It sounds like you have a blown head gasket. Combustion gases can enter the cooling system through either a blown head gasket or cracked cylinder head. The gases (under pressure) force coolant out of the reservoir. This also causes a void in the upper part of the cooling system , leaving no coolant at the thermostat and the coolant temperature sending unit. The result is a erroneous temperature Reading and will even prevent the thermostat from opening.
it is a hyndai that's why it is a hyndai that's why
The thermostat is not opening up enough,
You have to install the thermostat with the jiggle valve in the 12 o'clock position.
You could have a bad thermostat, just because its new doesn't mean its good, TRUST ME ON THAT, or you could possible have an air pocket in the cooling system.
The thermostat is inside of the water inlet on top of the motor. Disconnect the top water hose, then unbolt the water inlet. The thermostat can then be changed.
The thermostat on a Chrysler PT Cruiser is changed by disconnecting the upper radiator hose, unbolting the thermostat housing, and removing the thermostat. A new unit can then be put in place and reattached.
The thermostat on a 2004 Chevy Venture is changed by draining the radiator, removing the upper radiator hose, and unbolting the thermostat housing. The old thermostat and gasket can then be removed and replaced.