Subarus are notorious for the system having air in the lines and if your thermostat is on the bottom of the engine a weird on thats on all the 2.2 i have owned if you have opened your cap and the system was just filled or antifreeze changed unplug your electric fans i would say put a new thermostat in first they are under 10 bucks better save than sorry with the fans disconnected it will get hot quicker it will also open the cold water at the bottom to open the thermostat your pump will circulate the water keep adding till full throttle up engine it will **** it down add some more let off throttle the coolant will start to flow out cap it off shut it down reconnect the fans but the cold coolant in the botton stays cold till the top water opens it up the thermostat the entire system needs to get hot mix and circulate get most of the air out.

yes worst case you may have head problems lost coolant leak waterpump or line leaking head problem if your fluids are low it will overheat if all the coolant is there and you replaced your thermostat there is a fix to drive a while longer and thats just removing your thermostat it runs cold and if its winter take forever for your heater to warm up sometimes not enough hot water goes through the weep hole in the thermostat too the little pin gets stuck if you smell antifreeze in your exhaust then yes a head gasket they will sometimes still run strong and be leaking and burning it off in the exhaust... i had one like that actually the heads were bad i just swapped another engine i had sitting around

You probably need to change both the water pump and the thermostat. Otherwise check you're spark pulgs. If you've blown a head gasket, your vehicle will overheat. Pull the spark plugs and if you have one plug that's nice and shiny, it could be that you're leaking water into that cylinder and you've steam cleaned it. I would then do a compression test.