The minimum temp is set by a 195 thermostat. Anytime the radiator tries to cool the liquid down below that, the thermostat closes partially and restricts the flow. The radiator cap is like a pressure valve on a pressure cooker, keeping the liquid from boiling by raising the boiling point 3 degrees for every pound of pressure. a 15# cap would raise toe boiling point to 212 + 45 = 257 degrees.
Most modern electric fans turn on at 220 and off again at 200, so if a cap is unable to pressurize (leaky gasket, bad spring, not turned down tight, etc.), the coolant can boil away before the fan ever starts running.
Jerry
