Fern's right, but don't just do the IAC passage. Use a 3M brand 'Throttle Body Cleaner' and an old tooth brush. With the engine off, block the throttle wide open and clean away all the black 'crud' you can see on the throttle blades and in the TB bores. You can also remove the IAC valve to clean the passage and the valve pintle (be careful not to get cleaner into the electrical part - hold the valve with the pintle down while cleaning).
In general, the idle stop screw does not control idle speed. The ECM does this with the Idle Air Control (IAC) valve in an air bypass passage. The idle speed is programmed into the PROM. The factory setting for the idle stop screw provides a base air flow past the throttle plates. The ECM commands the IAC open to add more air for a typical controlled idle speed of 750 - 850 RPM. If the IAC were closed down* or its passage were blocked, the closed throttle blades would allow a warmed-up engine to idle at about 500-600 RPM with no load.
A quick check for normal air flow past the closed throttle is to read the IAC counts with a 'scan' tool or program. At a warm idle with transmission in park and A/C off, the counts should be between 009 - 021. If counts are high (> 021), suspect low airflow past the throttle blades. This could be because they need cleaning, or they were 'adjusted' too far closed. If the counts are low (< 009), suspect extra air entering the intake from some other source besides the throttle (this could also be called a vacuum leak). If no vacuum leaks are found, then check the throttle for being 'adjusted' too far open.
If you find that someone has removed the plug and 'adjusted' the idle screw, you can get it close by backing the screw out until the tip no longer contacts the throttle stop. Then turn the screw slowly back in until it just makes contact. Finally, turn the screw in an additional 1 1/2 turns. Use locktite on the screw so it doesn't move with vibration. If the throttle stop was 'adjusted' by bending it, you should remove the plug and follow the above proceedure. If the screw is siezed, you'll have to bend the linkage. Make sure the throttle body is clean and that there are no vacuum leaks. Use a 'scan' tool to read the counts as you bend the linkage. Shoot for 015 counts.
* To completely close down the IAC, start with the engine at idle, and slowly remove a large vacuum hose (ie. power brake hose). The engine speed should increase over 1500 RPM while the ECM tries to control it by closing the IAC air passage (counts go to 000). Next, unplug the IAC electrical connector. When you slowly plug the vacuun hose back in, the engine will slow down to it's 'base idle' speed because the IAC can no longer add air to that which goes past the nearly closed throttle blades.
