04' 3500 5.9 manual. Truck fires up perfect and idles perfect. When you head down the road and come to a stop sign or light the truck will idle fine and then drop 500 rpms and then back up etc... until I give it a little gas and the idle comes back to normal and it will be fine until the next light. Any guesses what may be causing this?

I know its not the filters as I changed them all about 500 miles ago.

 

Sounds like the FCA needs replaced. $100 from Cummins and about 15 min. should do the trick.

 

I think I have one battery that may need replacing. The little light is jet black. Could the battery be pulling the idle down at stop?

 

I suppose in theory it could be a battery... could be stalling the pump out. What's you're voltmeter read when you stop?