Subaru draws vacuum on cooling system after running hot

if there isnt any leak in the cooling system the fan clutch is changed and the fan along wit the thermostat dat mean u can probly have a clogged radiator with poor cooling system circulation or u might have to take it to a repair shop to check if the head gasket is blown

2 possible things. 1st(cheapest) you have air in cooling system. Need to burp it. 2nd(expensive) you may have blown head gasket. I tend to think air is in cooling system and your level is low.

This is either a cooling system fault which could be, thermostat, radiator or fan thermo switch, check that both top and bottom hoses are hot, if one is cold the thermostat isn't opening, check to make sure the radiator fans are turning on. If the thermostat is opening and the fans are turning on you need to take the car to get the cooling system pressure tested as you may have a leaking head gasket.

Answer could be a faulty thermostat,or it could be a sign that your head gasket is about to blow ,or last of all your timing could be out of sync.

Answer 2 from Inventus: It means your cooling system is funtioning properly. In a system having a coolant recovery tank, coolant in the radiator is always up to the brim, hot or cold. There is negligible or no air space. When coolant in the radiator expands sufficiently due to warming from the engine, it will squeeze past the pressure cap's bottom seal and flow into the recovery tank. (If no provision for such expansion was present, the expansion would rupture the radiator or your hoses.) Only coolant within the radiator is under pressure, and because of this pressure (together with the elevated boiling point that the "anti-freeze" permits), it normally does not boil. But once past the pressure cap's bottom seal, the overflow is at atmospheric pressure and therefore boils.
This boiling is usually unnoticed after a short, i.e., local, trip because the cooler coolant already in the recovery tank quenches it. But after some highway driving the influx of more hot overflow heats up all the coolant in the recovery tank to the (unpressurized, i.e., "natural") boiling point.
As the engine cools when shut off, the contracting coolant in the radiator sucks back coolant from the recovery tank. Fluid in the recovery tank should never be below the "full hot" or "full cold" marks, lest air be sucked in.
-- BETTER ANSWER ==
Your cooling fans are not turning on. It is not normal for your overflow tank to boil like that. It is true that your radiator is overflowing into the reserve tank, but that means yourr adiaotor is boiling. Check for blown fuses or relays for your cooling fans. IF theya re fine. run your engine for about 15 minutes and drive. When you temp level is at normal operating temp open your hood with the engine runing and see if your fans are on. If they are, then you may have a bad thermostat or a plugged radiator, or a bad water pump. If the fans are not on, get your cooling fan switch replaced if your car has one. Check your temp sending sensor

REPLACE THERMOSTAT AND RADIATOR PRESSURE CAP.CHECK MECHANICAL COOLING FAN.SHOULD HAVE RESISTANCE WHEN TURN WITH HAND.IF FREEWHEELS FAN CLUTCH NEED TO BE REPLACED.CHECK TO MAKE SURE AUXILIARY ELECTRIC COOLING FAN RUNNING HOT WIRE COOLING FAN IF FAN RUNS.YOU HAVE FAULTY THERMO SWITCH LOCATED AT THE BOTTOM OF RADIATOR ON THE LEFT HAND SIDE ON 4 CYLINDER ENGINE FOR V6 ENGINE THERMO SWITCH LOCATED ON THE LOWER RADIATOR HOSE ON THE RIGHT HAND SIDE OF THE ENGINE COMPARTMENT. CHECK FOR BLOWN COOLANT FAN FUSE OR FAULTY RELAY.IF THE AUXILIARY ELECTRIC FAN DONT RUN WHEN HOT WIRED THE ELECTRIC FAN FAULTY.

you either have a closed thermostat or a blockage in the radiator

What are you going by to say it's overheating , temp gauge ? Have you test to see what the actual temp. is > Hook - up a scan tool ,what's the PCM - engine computer seeing for engine temp. ? 223 degress the PCM should turn on the cooling fan . You could buy a Non-Contact Infrared Thermometer With Laser Targeting at harbor freight ! Radiator very hot ????? What is the right temp .
Non Contact Infrared Thermometer With Laser Targeting

Could be a bad temp gauge . Is the coolant surging inside the cooling system ? Does it have a radiator cap ? Have you replaced it ?

It could be the fan temp. switch but it shouldn't have been boiling in the overflow tank. I'd recommend doing a compression test to see if you have a blown head gasket. If it's a blown head gasket, the compressed air/fuel mix being force into the cooling system could well look like boiling in the over flow tank.

Is the cooling fan running? If so, drain the radiator with the heater control set to hot. Install a new thermostat, and fill the system with a 50/50 mix of antifreeze and distilled water. Bleed all air from the system by using bleeder screws if equipped, and by starting the engine with the radiator cap off radiator full. Watch for escaping air bubbles and keep the radiator full. When you see no more air escaping put the cap back on. If the fan is not running then that is your problem. Turn the A/C on and the fan should run continuously. If it does not the fan motor is more than likely bad. Check the fuse for the fan. If it is good, Apply 12 volts directly to the fan motor to check it. If it runs then replace the thermal relay.

You say it gets hot and overheats, are you losing coolant when it overheats? If it isn't boiling over, you seemed to cover everything that could be wrong, the fan is working, if it is not boiling over, just registering hot on the gauge, I wonder if it's just a temp. sending unit that's bad, indicating a overheating problem when there really isn't one. The water pump is bad when the shaft bearings go bad, and it will leak at that point. So, let me know if the car is actually boiling over (steam visible, exiting the cooling system)