How do you know if you need to replace a heater core vs the thermostat?

Answer 1

You only replace the heater core if it's leaking. In that case, your defroster will deposit an oily film on your windshield. If you're not getting enough heat, either the engine isn't getting warm enough or the warm coolant isn't getting through the heater core. The thermostat controls engine temperature. If your engine takes a long time to warm up, replace it. (They're cheap.) Unless it's like ten degrees out, your engine should warm to operating temperature in five minutes. The temp guage should move up to somewhere in the normal range and stop and stay put at whatever temp the thermostat opens at. They usually stick open when they go bad. If it stuck closed you'd overheat in less than ten minutes. Or, the heater core can be clogged. Automotive water pumps don't push the coolant that hard, so it doesn't take much of a clog to impede flow. Warm up the engine, shut it off and feel the lines (hose or pipe) that carry the hot coolant through the firewall to the heater core. If one is cool, pull both lines and flush out the core with a water hose. Make sure you have good clean coolant of the right mixture and DON'T USE RADIATOR LEAK STOPPER!!!!!!!!! It should be called "Heater Core Plugger." The other possibility is that the little flapper under the dash that controls the air flow through the heater core isn't working.