First problem is, that thing you call and idle screw IS NOT an idle screw. IT'S NOT for setting idle rpm. It's the "minimum air bleed" screw. It's actually used to set the IAC value.
Second, Idle rpm is set in the firmware/software of whatever powertrain control you're using.
As you screw the air bleed screw out/back it closes the throttle which should cause the IAC number to increase, in an attempt to maintain the idle rpm as set by the ECM. Turning the screw in opens the throttle blade some, and will momentarilly cause an increase in rpm, but the ECM will see this and decrease the IAC numbers to stabalize the idle rpm at the ECM's set value.
Also, keep in mind you only make a couple of small adjustments to the air bleed screw before you have to re-adjust the idle TPS value as it will change when fiddling with the screw. You have to go back and forth between the two adjustments to get them right.
Old hat habit calling it an idle screw. I know where everything is in the bin, and what to change, and how it works, but I just don't remember where the minimum air bleed screw is supposed to be. I have to do a search, get that set, then I can go into the bin and adjust my commanded park and drive idle speed.
:EDIT: Found this.
IAC (Idle Air Control) Adjustment
The IAC function maintains idle quality through commands from the ECM, but, has
NO impact beyond the idle range. Idle speed is set by the chip, not by the
IAC adjustment screw. Looking at the
inside of the throttle body, there are two holes in the lower portion fore and aft of the throttle blade. At idle, the blade is essentially closed and air enters the front hole, goes past the IAC plunger, and exits behind the throttle blade into the plenum. The IAC plunger is pulsed by the ECM to maintain a steady idle with varying engine load.
When the IAC is adjusted, we seem to typically look for IAC counts on our scan tool somewhere between 15-25
when the car is in Park, the engine fully warmed up, and the A/C is off.
The lower the IAC number, the less control the ECM has over the idle as the throttle blade begins to be opened. This setting may not be as critical as we often make it. You may find your car idles just as well at 40 counts as it does at 15.
With car in Park, engine
fully warm, A/C off, look at the IAC counts on the scan tool. If you wish to lower the count number, turn the
adjustment screw clockwise. To increase the counts, turn it counterclockwise. Turn the screw a small increment, turn the engine off, and restart. This insures that the IAC resets and confirms the adjusted number. Continue until the desired number is achieved. Often, on stock set ups, about 1-1 1/2 threads of the adjustment screw will emerge thru on the lever side of the throttle body. Restarting also rezeros the tps as stated in the prior section and removes any effect on idle speed that may have occurred due to tps movement. The IAC counts will probably be 30 counts, or more, higher on a cold engine than on a warmed up engine. The counts will also be much higher in gear as compared to Park, and, higher with the AC turned on.
Remember that the IAC adjustment will change the TPS and that if the TPS moves past 0.46 volts, the idle may increase in speed as the ecm no longer thinks the car is in idle range. Therefore, if you are going to decrease IAC counts very much, it is a good idea to first lower the TPS down to 0.38 volts or so in order to prevent it from rising out of the idle range as you adjust the IAC.
It is not a bad idea to clean the throttle body out periodically with carb cleaner to keep the passages clean and to insure that the IAC function works correctly. Remove the IAC from the housing and clean any carbon or gum off the tip of the plunger and clean the seat as well. Don't power up the IAC when the unit is not installed. Otherwise, you may find the plunger is pushed out of the sensor. When reinstalling, very little torque is required. Just tighten enough to compress the gasket to prevent an air leak. Over tightening may crack the plastic interior of the IAC.
