97 subaru outback 2.5 over heats going up hill at about 3000 rpms

Make sure that the radiator is not restricted. Using a inferred thermometer make sure all the cooling tubes are cooling. Make sure there are no air restrictions between the condenser and radiator.

It is a black plastic unit with a small wire harness going to it. The unit is located on a heater hose close to the heater core near the firewall.

its under the glove box facing up. you remove it by dropping it down.
there should be 2-4 screws and the drain hose

hi sounds like the throttle position valve they can cause this sympton when getting worn

Try running the car's interior heater at full blast while bleeding the system.

On almost all cars if you follow the upper radiator hose to where it goes into the motor, that is where the termostate is.

PROBLEM COULD BE FAULTY IDLE AIR CONTROL SOLENOID VALVE ALSO.CHECK AIR BYPASS HOSE FOR CRACKS OR DISCONNECTED BROKEN.

A FAULTY THERMOSTAT STUCK OPEN WILL EFFECT GAS MILEAGE AND CAUSE POOR OR NO HEAT CONDITIONS.THERMOSTAT NOT A HARD JOB TO FIX.ALL YOU NEED TO DO IS DRAIN OUT SOME OF THE COOLANT IN A DRAIN PAN.DRAIN COOLANT LEVEL BELOW THERMOSTAT.IF YOU HAVE 3.8L ENGINE THERMOSTAT HOUSING ON TOP INTAKE MANIFOLD.IF YOU HAVE 5.3L ENGINE FOLLOW TOP RADIATOR HOSE IT WILL LEAD YOU TO THERMOSTAT HOUSING.REMOVE HOUSING THERMOSTAT INSIDE THE HOUSING.IF YOU HAVE WATER INLET ANDTHERMOSTAT IN ONE COMPLETE UNIT OR HOUSING YOU HAVE TO REPLACE THE ENTIRE WATER INLET HOUSING WITH NEW THERMOSTAT.THIS TYPE UNIT WILL HAVE LONG PIPE WITH BYPASS PIPE WITH WATER INLET WITH THERMOSTAT. ONCE YOU REPLACE THERMOSTAT.IF YOUR RED ARROWS OFF AND HEAT STILL DONT WORK THE DEALERSHIP WILL HAVE TO CHECK IT OUT.WITH NEW THERMOSTAT AND PRESSURE CAP.MAKE SURE GET COOLANT LEVEL CORRECT.SEE HOW HEAT WORKS.IF NOT YOU HAVE A ELECTRICAL PROBLEM AT AC AND HEATER CONTROL SWITCH.

It could be the water pump, radiator, or cooling fan not working properly. If your heat is off it will not be the heater core.

check the coolant level - you may have low coolant, causing the sensor to occasionally read an air pocket. If you don't correct this you will burn out your motor.
We had this problem & eventually replaced both the thermostat and radiator cap, being careful to bleed out any air pockets by running the engine form cold with the heater on hot and the radiator cap off - keep watching & topping up the coolant as the enigine heats & the thermostat opens.
Good luck!