I have a new issue will my 03 Chevy 6.0. When the truck is cold it idles rough. Doesn't actually feel like a misfiring cylinder. In gear at a stop light the idle will drop below 500 rpm and the engine...
Remove the idle air control valve and thoroughly clean it and the cavity. A sticky valve with respond to the ECM properly but not physically move. So, you can get odd idle behavior like you saw but never trip a check engine light.
My FIL had a chevy van that was racing at idle. He could go 45MPH and never press the throttle and had no check engine lights. I replaced the AIC and all was good.