my truck (new to me project) has been idling low. I know you're not supposed to adjust the idle screw, but I figured if I get it to stay at 500 rpms, it's a start. well, the screw is gone. any idea what fits in there?

 

Anti-diesel screw is what that part is called

Might have to try a few size to find one that fits, or try wrecking yard
When you adjust this you are also adjusting TPS(throttle position sensor) so heads up on that

IAC Valve is what controls the idle on a fuel injected engine, yours is not working or unplugged.
Check that first.

On cold start do not touch the gas pedal
Start engine and it should REV to 1,500+ RPMs then drop down to about 1,100rpms, that's the IAC Valve working, and the ECT Sensor, if yours doesn't do this then IAC problem

As engine warms up computer starts to close IAC valve bit by bit to set warm engine "target idle" 625rpm for manual trans, 750 for automatic