I have never had this happen with any of my cars.

The 32 I just bought was running very cool at 160 or so. The guy I bought the car from literally had 100 miles on it and and never really got it hot or in traffic.

I was working on it, while letting it idle in my drive for 15-20 minutes. The temp guage only showed 180 temp. I shut it off and startee to walk away and it started blowing air and a little antifreeze out of the over flow tank AND the radiator cap was allowing (again mostly air/steam) out! The engine did not seem overly hot either!

When I drive the car it runs great at around 160 with no problems at all. This just happened when it sat at idle for 15-20 minuytes.
Right now there is no thermostat in the intake but I plan to put one in.

Thoughts on what may have caused this and what I should try?