Given ,
wrote:

What's the boiling point of unpressurized 50:50 coolant?
About 225F at .96 bar

Thanks Clare and Ed, for those numbers (where Ed said 223F which is close
enough to 225 that I'll use the 225 figure).

So, with the cap off, the 50:50 coolant boils at 225 degrees.
With a pressurized system, Ed says we can go to 250 before boiling.
The thermostat has a 180 degree opening point.

If I assume that the coolant is designed to be at around 200 degrees, plus
or minus 10 degrees, that means an unpressurized system should work just
fine for light loads since the coolant stays below the boiling point of
water, at least on average.

I think what was happening was that locally, in hotspots, the coolant was
boiling since it was no longer pressurized.

Does that logic make sense?