OK, I have a 2009 Chevy Silverado with the 6.0L under the hood. Question is, what RPM should the engine idle at? Right now it will hover around 500 rpm, but every so often the engine will stumble while I'm sitting at say a red light and drop below 500 rpm. When it does drop below 500 rpm it's not that far, we'll say around 485 (just guessing because the gage increments and decrements in 500 rpm stages). I hope all of this makes sense. Thanks.