Silverado Cold Start very low idle

[ksack001]

210 Silverado 1500, 5.3. Ok so recently my truck has been idling very low on cold start. It’s supposed to idle at like 1500 or so then drop down as the engine warms. For about 2 weeks now when doing a cold start after sitting all night it idles only at 500 the normal idle speed. If it sits for only 5-6 hours and I start it, it has the high idle like it’s supposed to then drops back down when warm, but when sitting all night it starts up fine but there’s no high idle until warm like it’s supposed to. There are no codes at all and my MPG are way down. Only thing done to the truck is a Carven Cat back, Range Device which has been there a while, new battery and KandN filter. I’m at a loss here. I’ve changed the ECT, and have unplugged battery to reset ECM, but nothing. I have a new MAF sensor coming but I’ve cleaned the current one. Any ideas? 

[CamGTP]

Mine does this with E85 sometimes. It also takes a while to start on E85 when it's cold, so it cranks for a little bit extra. If I shut it off and start it right away it does the normal cold idle rev down.