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2003 Ford Mustang High Idle when I start

I have a 2003 Ford Musang 3.8L v6 when I start my car for the first time of the day or after several hours of sitting the car idles high between 1000-1300 RPMS for appoximately 15-20 seconds after that it idles down to 600-800 rpms. I changed the spark plugs wires air filter not sure what else it could be no Check engine light. Any help would be appreciated.

It sounds as though the system is operating just as designed.