Subaru Forester 2.0 ’09 Overheating

Hello guys,

I have a Subaru Forester 2.0 (sh5) y.2009 with overheating issues. About a year ago, the engine was replaced with a used one (~70,000 km), and the timing belt and water pump were done at the same time.

The problem started couple of weeks ago with overheating during highway driving. The temp warning light comes on flashing, and it only goes back to normal if I turn the cabin heater on full hot. At that point, only the left side vents blew warm air, the right side stayed cold (it has been doing this for a while).

I replaced the thermostat, radiator cap, and flushed the heater core both ways, with no effect. I also bled the system with a funnel kit and no more air bubbles come out with the cap off.

It still overheats on the highway, but cooling improves with the heater on. Only about half the vents blow warm air. Both upper and lower radiator hoses are hot, but the radiator fins stay cold.

Could this be a clogged radiator, or should I suspect a head gasket or another coolant flow issue? I also tried bypassing the heater core, but it made no difference. Actually made it worse since there was no extra cooling via heater anymore.

Any ideas on how to properly diagnose this?