Turning up the heater will cool the engine

In the summer, if you see your coolant temperature gauge creeping up, that means your engine is over heating.

Immediately turn the heater on. The excess heat goes into the heater core, which is just a tiny radiator, pulling the temperature of the coolant down. Of course, it will rapidly overheat the interior of the car — and you. If doing this means getting to the top of that hill without boiling over, it might get you to someplace that can check the cooling system for problems. If you need to use the heater frequently to reduce coolant temperatures, either there's something wrong with your cooling system, or you've seriously overloaded your vehicle.