Ford Repair: Taurus high idle, air control valve, liter dohc

Question
I have a 1999 Ford Taurus 3.0 Liter, DOHC with about 79,000 miles and it idles high sometimes. It is unaffectd by weather or how often the car is started, as far as I can tell. Sometimes it will idle at 1500 RPM for 30-45 seconds, sometimes it will idle at 2500 RPM for several minutes. The fuel filter was replaced last month (for unrelated reasons), and I replaced the Idle Air control valve also. Regular maintenance is performed as well. Normal idle is around 900 RPM. Any ideas?

Answer
Hello and thanks for the question...I'll try to help.
I would suggest that you check the computer system for codes. Even though they might not light up the check engine light--they will be useful to getting your problem resolved.
Hope this helps,
Erik