What are your rpms at? I've heard that they should be about 720 but i would have to look at my scanner and see for sure. I've heard some guys uping the idle to 765 and some lowering it to 680 and 650.
I know the 2011 and up dmax will idle higher than that to warm up like the 800-900 range but will drop when you hit the brake to place in gear and i find mine will idle up while parked after I've driven it a while, im assuming to help cool it down. ( and im not talking about the actual high idle switch being engaged that is used for cold weather ).
I haven't had that generation of the duramax motor but ive had the 2nd gen (2005) and the current (interim 2015 ).
Higher idles are actually safer than low idles on bigger engines' bearings from what I've heard.
Sorry if this doesn't help much.