A buddy of mine has an '03 K3500 CC D/A. The truck has about 20K on it. He recently returned from a trip to Missouri and noticed that and area directly below the fuel fill is bulging outward. Not just a bit, but quite noticably. I have never heard of a thermoset plastic reacting with diesel like this. Has anyone else has this problem or even heard of it? He is taking it to the dealer next week to see if they know what might be going on....

I never really looked but since you say plasitc, the fuel fill door on the 3500 is in the dually bulge, not in the sheetmetal on the side of the truck? If so I could see the diesel spilled on it causing some warping maybe, but a big bulge is odd. At least it's under warranty still...

If this is the case of diesel causing this I would think it would be a recall, unless recall are only for safety items.
I am not that familier with the dually set up but do the sides have a couple of bars on the inside of the fenders holding them in place, and could it have been bent by somthing.

Nope, no supports........The area directly under the fuel door is bulged out about and inch or so.....Right under the spot where you would normally drip some fuel......