Vehicle Owner: The symptom which you describe could be due to low engine coolant level.
The coolant level in the coolant reservoir is not always an accurate indicator of the
coolant level in the cooling system. Any leak in the cooling system may allow coolant
to boil off out of the radiator without lowering the level in the reservoir. visually check the coolant level in the radiator by removing the pressure cap, ( with a cool engine only ! ).