Bad radiator cap?
Rad cap does two things, it allows pressure to build up in the system as coolant expands, this extra pressure raises the boiling point of the coolant inside.
Above this preset pressure the cap allows coolant to expand out of the system and into the overflow tank, most cooling systems run 13-18psi of pressure.
After engine is off the cap has a second valve that allows coolant in the overflow tank to return to the cooling system as coolant contracts as it cools.
Do you have a coolant leak?
A leak does two things, it leaks coolant of course, but after engine is shut off and pressure goes down it can suck air into the system.
Air in the cooling system can form a pocket that will restrict flow, as coolant gets super hot if will vaporize, which raise the pressure much higher and very quickly so fast overflow.
With engine cold and rad cap off start engine, coolant level in rad should be full and no coolant should overflow while engine is cold, if you see a lot of bubbles and coolant starts to come out you may have a bad head gasket.
Let engine run for a minute or two if level in rad drops add more coolant, you had an air bubble, it is out now, well most of it
Rad coolant should start to warm up within 5 minutes, warm rad coolant means thermostat has opened, if rad starts to overflow after 3 to 5min. it could mean the thermostat is stuck closed.
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