To find out if your heater core is clogged, feel both heater hoses (the two hoses hooked on the heater core inlet and outlet at the firewall) with car idling at normal operating temp and heat set on high. Both hoses should feel very hot. If one is cold or luke warm, your heater core is probably clogged. Flush it or replace it.
If you have a heater control valve on your car, make sure it's not stuck close. If you have one, it's probably in one of the two hoses with a vacuum hose hooked to it.
Also make sure your level of coolant is good and make sure there's no air in your cooling system (bleed it).
December, 6, 2008 AT 4:53 PM
Well I couldn't believe it in the least. I did the diagnostic as you said. One heater hose hot the other heater hose barelyt warm. So I just replaced the heater core.
30 minute job the heater is now like I always new it to be. Hot on all sides. Could probably flush the old heater core and use it again. The thing is copper the one I replaced it with is aluminum. But again no more than the cost guess I could just junk the old one.