My car runs like a champ. But when it's warmed up, and I am sitting in traffic it idles at 1100 rpm. I'm getting 19 to 20 inches of vacuum and couldn't find any hoses loose, so it's not a vacuum leak. But here's the deal, the hose from my one way check valve(the aluminum one) in the fuel evaporative system popped off yesterday. When I reattached it it idled rough and low. I checked the valve and it was flowing in both directions. I replaced it with a spare green and black valve. Could having the wrong valve make it idle high? After I let the car sit a while and start it it idles normal, but if I blip the throttle it will go back up. Then if I turn my fans on for just a second and then off, it goes back to normal. WTF!!

 

By the way, my engine has 20k miles on it with a downpipe and homemade intake.