As a general rule, the temperature of the coolant in a car should be less than its boiling point. Because different types of car engines operate at different temperatures, the ideal coolant temperature also varies. In normal running conditions, the thermostat will control the cooling fan to keep the coolant temperature within the safe range. But the temperature can rise above that if the thermostat or the fan is not functioning properly. The heat output can also go above what the radiator can manage if you rev too much.