I have a 97 Legacy. It recently cracked the tank on
the radiator. Replaced the radiator, then it started
overheating when being driven. The coolant
backflows into the reservoir. I replaced the
thermostat and water pump. Still had issue. Took
off the hoses to heater core and flushed them, no
blockage. I've added over 10 gallons of coolant,
radiator seems empty when cold. Checked oil for
signs of cracked head/blown gasket, no water
present. No water leaking inside car. No water
leaking under car. Can't figure out where coolant is
going, or why car is overheating. Any suggestions?
It's probably being burned by the engine you need to do a cylinder leak down test, there's a crack in the head or a head gasket has gone bad, less likely is a cracked block.
Check passenger side floor to see if it is wet heater core maybe leaking.
I still suspect the head gaskets. Just because you don't have coolant and oil mixing does not mean the head gaskets are not leaking.