The idle RPM is controlled by the powertrain control module (PCM), which is the computer that controls the engine and automagic tranny.
Your normal low idle RPM is 680 RPM with auto tranny or 650 RPM with stick shifter. But in cold weather, after a minute or so, the engine will idle at a higher RPM, depending on how cold it is. Click here
for some discussion on the cold ambient pkg (CAP).
So the only way to change your idle RPM is to change the computer program in your PCM.
One way to do that is to install the Ford auxiliary powertrain control module (APCM), which is the brains behind the Ford auxiliary idle controller kit (AIC). However, that APCM defaults to 1,200 RPM as the lowest pre-set idle RPM. You can manually set the idle RPM anywhere between 670 and 1,200, but it takes some fiddling with the APCM every time you want to set the idle RPM at less than 1,200.
The only other way is to have a computer programming guru go into your PCM and change the program in the PCM. That's do-able by someone like Jody at www.dp-tuner.com
. If you prefer your low-idle RPM to be, for example, 750 RPM instead of 680, then Jody can do that.
Of course, if it's cold enough for the CAP to take over, then your idle RPM will probably be higher than whatever low idle RPM you choose.