[Xist]

1. Flush your radiator.
2. Disconnect the hoses to the heater core.
3. Connect the heater core that you want to test.
4. Fill the radiator with 100% water.
5. Run the engine for a while and maybe drive around the block. Bleed the air out of the radiator the best that you can, but don't push too hard, you are not running the proper mix!
6. Let everything cool down, disconnect the heater core, and replace if necessary. If for some reason you are testing in another car, reconnect the proper heater core, flush the radiator, and fill with the proper mix.