I believe that my heater core is about stopped up. Before trying to change it out what should i do to try and un stop it? Run the water hose through it both ways several times or blow some compressed air through it? How much air would be to much? Just some thoughts as i do not really want to change it out right now. If i drained the coolant could i put something in the water like citric acid to help clean the heater core out? What do places do when the flush the coolant system and would this help? Like said i do not really want to mess it up where i have to change it right now, for i can live with the barely warm air till i fel more like changing it. The inside of the radiator looks pretty good from what i can see but does not look as if the previous owner has just changed it or anything.