Hey, thanks for your detailed post post, here are some insights: Au contraire, as you probably know you can't heat coolant past its boiling point, which at normal sea level is around 100°C. At that point it would usually turn to steam and bubble away from the heat source. However, the boiling point massively depends on the pressure being exerted on the water, which means that you can raise the boiling point (and therefore the max liquid temperature) by pressurizing the system. That's exactly what happens in a car, the coolant system is capped off, allowing for pressure to build up when it would usually just turn to steam around 100C. The problem now is that we build ourselves a pipe bomb... That's why the coolant system has a max working pressure before a safety valve opens and vents pressure to the outside world.As soon as the coolant leaves the pressurized system, its temperature is now way over the (new) boling point and it turns to steam immediately. (That's where all the steam comes from when you overheat the car, both IRL and in the game) The evaporation happens outside the cooling system, right at the safety valve, so in practice this really doesn't cool anything. Heat transfer is indeed 0 if you have nothing to cool with. If you boil off all the coolant, there is nothing left to work with anymore... Making the heat exchange rate 0. (When it comes to the simulation, we are only looking at liquid coolant, the steam now in place of the former coolant technically can transfer heat too, but it's such a chaotic and irrelevant effect that we are not modelling that) Once you boiled off all the coolant, these numbers pretty much become useless, for math reasons there is always a tiny little bit of coolant left to prevent division by zero issues, but the whole coolant system is basically dead at this point and not in a surveillable state anymore.