Engine Cooling problem
2004 Kia Optima 6 cyl Front Wheel Drive Automatic 101000 miles
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I have a 2004 optima Ex 6 cylinder. When in traffiic the temp gage moves past the halfway mark and I put the car in N and reve the engine and the needle moves slowly back down. This does not happen if I have the A/C on, the temp gage stays like its supposed to. I have replaced the thermostat, and it was fine for about 150 miles, then started again with the gage moving when Im in traffic. does anyone know what the problem is?

thanks,