So here is the deal. Truck does ALOT of sitting IE less than 25k miles in the last 2 years. My little commuter car had problems so I started to drive the truck back and forth to DC everyday which is 120 miles round trip. Anyways, I noticed that the truck started to backfire alittle when you got off the throttle after a hard acceleration IE onramp doing 65-70 mph and I assume the MDS is kicking it at that point. Truck threw a CEL light and scanned it and it turned out to be a P1408 code. Figured it might be clogged up, so I pulled it off and cleaned it up. Put it back on and did it again, this time I replaced it and put brand new plugs in it. Doesn't do it as bad but it still does it. What can be causing this??? Truck has excellent power with no missing either at idle or speed. Thanks guys.

 

P1408 is an EGR code. Installing new plugs doesn't solve that problem.
http://engine-codes.com/p1408.html

Also, the backfire is certainly not from MDS.

for the record. My truck does more sitting than yours does...lol
2008 w/12kmi ;-)