Alright ht im gonna give you the run down real quick. I had a b20b in my ej8 that had blown a head gasket a mixed oil and water in everythin, while I had the head off I put a gsr head on it and used the stock water pump and timing belt the car was down for a couple weeks. I forgot to flush the heater core and it stayed clogged whenever id crank it no hot air would blow and the car would overheat, if I bypassed the heater core it would run at operating temp. I assumed I needed a heater core so I changed it, all new hoses, new thermostat, coolant and bleed the system thinking it would all be okay but the car still overheats if thr new heater core is hooked up and wont really blow hot air, maybe warm for a minute but then gets ice cold. I have no idea where to start next was hoping someone could give me some insight, thanks.

 

Did you bleed the coolant system with the heat on, allowing coolant to flow through the core?