2000 chevy silverado 1500 4.8 idles at 750 in park but when put in reverse it goes up to about 1100 and back down when in reverse to 600 same thing happens when going to nutrail and drive and same happens when going back to park. ittrips a po507 code higher than expected idle

 

It could be a bad IAC valve, a bad temperature sensor, vacuum leak, etc. Please read the following for a better idea of what needs to be performed in order to diagnose the cause:

https://www.700r4transmissionhq.com/...evy-silverado/

https://www.silveradosierra.com/vort...g-t325201.html

Good Luck!