On Saturday, August 5, 2017 at 12:19:24 PM UTC-4, Bram van den Heuvel wrote:
Given ,
trader_4 wrote:

If it's a minor leak and local, easy driving, I would do it.
Crossing Death Valley pulling a trailer, probably not.

I agree that going up a mountain or crossing a desert or a long 100 mile
trip might be pushing my luck, but I'm asking technically what happens if
we drive, essentially, with the radiator cap off.

Assuming everything else is working fine, the only difference I can think
of is that the pressure of the system above ambient is zero psi compared to
about 1 atmosphere.

That lowers the boiling point of pure water to 212 degrees, where a 50:50
mixture of coolant would be something higher than 212 degrees.

But how hot does an engine typically get?
Isn't an engine coolant generally around 200 degrees F?

Modern engines run hotter, I'd say they can easily be close to 212F.
That's why you need 50 50 antifreeze. And that's the overall average
temp, not the localized temp in the hottest spots in the engine.