high engine rev on 2003 Mitsubishi Galant

when I first start engine the engine revs up for about to 1700 rpm for about 2 to 3 minutes and then starts to rev down to a normal rev. My mechanic has not been to identify issue.

Asked by for the 2003 Mitsubishi Galant
Hi Ebony,
Did you every get this resolved? Do you know if there were any diagnostic fault codes stored?