[headgasketenthusiast]

Couple days ago I noticed that when I was driving my 02 and came to a stop light, heat on full blast would go cool. Immediately after the light turned green and started driving again the heat was back to normal. This has happened a couple of times since. Heater core going out? I installed a new aluminum koyo radiator about a month (2200 miles) ago and everything went smoothly, burped the system and haven't had any issues since. No overheating or leaking or anything weird, just heat blowing cool.

 

[XJman]

You completely flushed the coolant and didn't mix types?

I'd suspect a heater core clog, but I'd investigate why. It's uncommon on aluminum block aluminum head vehicle.

 

[headgasketenthusiast]

You completely flushed the coolant and didn't mix types?

I'd suspect a heater core clog, but I'd investigate why. It's uncommon on aluminum block aluminum head vehicle.

Yep. Completely drained and refilled with OEM Subaru coolant.

 

[SD_GR]

Heater cores are just cabin-mounted radiators. I'd suspect either an air pocket or a fault in the cabin air selector system. Pressure test the system to be certain there are no leaks, and go from there.