Subject: cooling system
Question: I own a 90 mitsubishi eclipse non-turbo 1.8L. Have you ever had heater problems? I have been having problems with my heater not being very warm when it is pretty cold outside. The thermostat is pretty new and the guage shows straight up just like it always has. I had a free radiator flush coupon from a dealership and they said they hooked it to a machine, but it did not seem to help at all, so time to do it myself. I assume there is something in the heater core that is clogging things up. I can hear the temp flapper working when going from cold to hot. What cleaner would you recommend to use and how the best way is to go about it. Any advice would help. Thanks, Landon
Answer: Landon,
You are most likely correct to assume a problem with your heater core. You may have debris built up on the fins of the core that is not allowing enough heat to get through the ducts. It's relatively easy to get to the core and inspect it to see if there is any blockage on it. If you want to clean it out internally, here's a couple steps to do.

-Drain your coolant by a quart or so. Catch it in a clean container so you can reuse it.
-Disconnect the 2 hose going into your heater core.
-Flush it with water. Try to get as much pressure through it as possible. There's no easy way to use any kind of cleaner. Water should work good enough.
-Re-install the hoses and refill your coolant to the proper level.

If that doesn't help, your flapper door may be letting in too much outside air. You mentioned that you can hear it, but it may not be closing all the way to prevent the cold, outside air from getting in. You may want to take off your lower dash panels to visually inspect if that door is opening and closing fully. I would recommend getting a service manual for your car for easy reference. You can get one at just about any auto parts store for about $15.
Good luck!