The engine is controlled by the PCM (powertrain control module) or ECM (engine control module) either way , it takes inputs from the many sensors in/on the engine to determine what values the 'controlled' items should be at. IF it thinks the engine temp. is -40 degrees , it will adjust the air/fuel mixture accordingly , and will typically be at a higher RPM until it sees the temp. coming up. An intermittent fault (NOT necessarily a part)in the system may cause the symptoms you describe. A SCANNER can be hooked up and be used to watch all the sensors and determine if any one or more is out of range , that would be done while it's running , preferably from a 'cold' start (after sitting overnight). You can find inexpensive (relative term) that would work for your purpose in the ballpark range of probably $250.00-$500.00( I'm guessing based on some of the stuff I've seen at places like 'autozone')Professional ones start at several thousand , so , maybe(just maybe) that helps in understanding why places that do more than just 'read codes' will charge a diagnostic fee for simply even hooking it up. My point to all of this , is not to go out and buy parts based on just a code or a guess at a common symptom (which is especially easy to do online with no accountability to your wallet). Even if you do the repair work yourself (still saving money) , hopefully it would be a one-shot deal, fixed right the first time. IF I was to confusing in my reply , let me know what I can clarify.
Hi friend (ziptie?) Thank you very much for the in-depth, quite thorough and informative response to my reply. I grew up turning my own wrenches when I got my first car in '65 but now having experienced a very significant natural change in hair color, I am not in the mainstream anymore regarding the computerization of systems and am not even too familiar with the jargon. You have presented yourself as patient and obviously very willing to help out, and for that I thank you again. I am sure you have, and will continue to assist many like myself in the future.
You're welcome. By the way , if by 'idles fast' you mean up to 2200 RPMs , AND it takes maybe no longer than 3 minutes to come down to normal idle(usually about 750 in gear), then , what you are experiencing sounds pretty normal to me. Maybe one day when you're bored and want to sit in your driveway for 3 minutes , set your smartphone (I know you gotta have one of them?) to 'stopwatch' setting and start the truck and the timer at the same time.....and just sit there patiently observing( another lost art), see where it's at at the 3 minute mark.Let me know. Have fun!....
Are you still having this issue? How long does it idle fast when you first start the car? Does this only happen when it is a cold start? Or does it still happen if you drive around and the engine is hot, turned it off and then turn it right back on. Does it still occur?