What would make my 'radiator overflow' tank spew hot water?

Could still be the thermostat. It could be stuck in the open position, which could cause what you describe. It's cheap and easy to replace most thermostats.

If the radiator was empty, that's enough to do it. Where did the water go? It leaked out somewhere other than the overflow tank. Find the leak: Worn out water pumps leak. Bad hoses leak- including heater hoses. Radiators leak when they have holes in them and heater cores do too- and you would smell that inside the car.

Water you don't see leaking may escape through a bad head gasket and depending on which part of the gasket has failed, the water may be going into the crankcase or through the combustion Chambers then out the tailpipe as steam. That kind of leak will create an emulsion deposit on the oil filler cap and/or on the rradiatorcap. Fixing that will be expensive and take a good mechanic, unless your engine has cracked its head or block. That would be more expensive than it's worth- having a cracked block and replacing it with a used engine would be better than trying to fix it..

A stuck closed thermostat will cause heat like that. A plugged up radiator will do it. A worn out waer pump will not cool the engine enough. Fan not working will cause that. All that is besides water having leaked out.

Never put cold water in a hot engine without it running, and pour it in slowly to prevent cracking the engine head or block. This can be hazardous too while you do it, because the water going in will turn to steam and spray back out on you if you don't watch out. It can give you 2nd degree burns so quick you have no time to jump. It's best to wait no less than a half hour before trying to refill an overheated engine.

If you take out the thermostat and run the car without it and it keeps normal temperature, then it was a bad thermostat. If you want good gas mileage you need a good thermostat in there. But usually after you get the kind of heat you got, more damage happens. Sorry. Good luck.

Sounds like a headgasket to me. When the head goes bad the cooling system actually purges coolant out of the engine, radiator then to the overflow tank. it has to escape somewhere so it pisses it out the overflow. to much pressure in the engine block. refill everything to specs. let engine idle for about 10mins then check for bubbles in the over flow. it you see them you know your head is gone. a temporary fix is to remove the thermostat all together. when you do this it will allow continuos flow of coolant through the engine. the down side is it will take your car longer to warm up. also you will have little to no heat in the winter. good luck