1999 Chevy Silverado code PO507

If you work the throttle by hand, with the engine running, do the rpms drop quicker. Can you tell if the throttle plate, is closing, and not being held open, someway?

If you start the vehicle and let it idle, without touching the throttle, are the rpms ok?

Can you check the signal on the tps? I 'm not saying, replace the sensor.

A lot of vehicles will idle high on cold start-up, then gradually go down as the engine warms up. I did see the code you listed.

On a fully warmed engine, is the idle still a problem? I'd want to see the iac counts and you need a scanner, where you can see live engine data.

If you have a scanner, it will show what the sensors are doing, or you can backprobe the dark blue wire at the throttle position and use a dmm, turn on the key, you don't have to start it, have helper work the throttle. The voltage readings should move smoothly, up and down as the throttle is moved.

Without at least a scanner, it's just hunt and peck. There are several things that might request a higher idle, if it isn't some kind wiring or mechanical issue with the iac system.

The alternator--hvac--power steering, just to name a few, may cause the computer to request a higher idle.