2004 Mazda 3 2.3 5 speed overheating only when heat in use.
 

Hello everybody. I have a '04 Mazda 3 5 door 2.3 NA. I have had a bit of a problem with it overheating. Now I know what you are all thinking. "Here we go. Another post about overheating". Well its not your typical overheating during the hot weather sort of thing. I live in Michigan. And the weather is starting to turn a bit on the chilly side during the early morning hours. So I start my car about 10 minutes before I leave for my job. The car warms up fine and the heat blows fine. And the heat works for about 15 minutes into my highway drive(70-80 mph). Then the problem starts. The temp gauge rises to almost hot. Mind you the heat is blowing hot right now. So I would stop and shut the car off for a bit until I realized something. It only overheats while the heat is set to hot. When I set it to cold, the car absolutely does not overheat...EVER! Its only when the heat runs for a short while while driving. I tested this theory this morning. It started to get hot so I turned the heater temp to cold. As I suspected, almost instantly, the temp dropped to normal operating temp. This is the exact opposite of what I have learned when a car starts overheating. If anybody has any suggestions as to what may be the culprit, please fill me in.

Update on the problem. I also forgot to mention that I have replaced the thermostat twice in a month as well as the water pump. Did a cooling system flush. I was driving home from work today and noticed something. If I run the heat on recirculate, it does not overheat. But if I have the fresh air setting on, that's when it overheats. As well as starts blowing cold after a while. https://cimg2.ibsrv.net/gimg/www.maz...3be7e82bac.jpg
The orange light is recirculate. The green light is fresh air.https://cimg0.ibsrv.net/gimg/www.maz...67e07cec46.jpg