I have a 2009 Kia Optima SX with about 47K miles. When the car sit idling (in Park) for about 15-20 minutes. the RPMS start to rev-up, there is no check engine light that comes on. I was wondering what that might be?

Hi, I'll make just a quick guess since there aren't many details with your post but normally cars don't just sit and idle for 20 minutes at a time. Your service manual should tell you to shut your engine off while stuck in a traffic jam but we all know that your never just sitting for 20 minutes your just inching along. The radiator in a car is just a heat exchanger used to cool your motor while driving down the road with air flowing thru it. When your stopped with no air flow there are cooling fans that the engine computer turns on and off to keep air coming thru your radiator. Your engine being small and holding just a little under 6 qt's of antifreeze can't circulate the coolant fast enough at idle for long periods of time to cool it properly, so once the engine controller see's the temp raising with the fan's on and even sometimes off it will automatically raise your base idle several hundred RPM to speed the flow of coolant thru the radiator by spinning the water pump faster. I hope this anwers your question.