Heater core 91 Mazda 626

I have a 91 Mazda 626 in which the heater and defroster does not properly heat up. I have fluched and filled the radiator and that worked for a day. The ford dealer says I need a new heater core. I am going to try to back flush the heater core to see if this works, is there anything else that I should check for or do if this does not work. Thank you for your knowlegable replies.

i was having hte same problem and i did the garden hose flush hooked up to the heate rcore hoses. that took care of it and it is working well. on another one i just had a air lock and took the heater hose off at the febder wall and circulated the water until it flowed throught hte other hose. good luck.