Any ideas on this? I got into it today, it's been running fine for months and all of a sudden in park it idles at like 500 rpm real rough... At redlights it idles at about 550-600 rough... Once you give it some gas it's fine... I was thinking about cranking up the idle a bit with that idle control nut under that cap... what do you guys think?

 

The Stepper Motor aka Idle air control valve being dirty is my guess. Had that same problem on mine and cleaning it up took care of it. Hope it helps.

 

What year is your truck and how many miles on it? By try to rest the idle instead of trying to find and fix the problem?
What octane fuel do you use, have you checked the stepper motor, have you checked for vaccum leaks, have you checked your pcv and cleaned or replaced it, have you cleaned your throttle body?
These are just some of the things you need to be looking at.
Your idle is controlled by the ECU, don't try and second guess it by over riding it.
Mike