1989 Toyota Corolla Sedan boiling in overflow tank no residue in radiator or oil dip stick. what could the problem be. Not finding any leaks, electric fan works.

You have a small headgasket leak from the combustion chamber to the water jacket and it is slightly pressurizing your cooling system ,it will only get worse ,it cannot get better,you will need to replace the head gasket ,but before you do take it to aradiator specialist and ask them if they would test your coolant for combustion gases .that will prove beyond doubt what is going on "youdoit"

Could be the thermostat is not opening up all the way and causeing the engine run hotter.
Water does go into the overflow tank when you turn a warm engine off. Thats why they have overflow tanks.