wel shouldnt never add to a hot motor at all and really better to add it while its running car gets hotter when you shut it off theres no fan blowing on it any more plus theres no coolant moving thru ity to cool next tiem car runs hot let it run pour water over radiator for a minute to help cool it down turn it off let it sit a hour crank an dthen add what ever an dsure theres alwasy a chance of damage but first was a problem to make it over heat to start with