My buddy at work has an 04.5 LLY 3500 single rear wheel duramax. His truck after warming up for a couple minutes starts to run rough while at an has blue smoke that come out of the tail pipe. Truck runs fine when on the gas just rough at idle like when at a light. It was brought to the dealer, and they replaced the number three injector and the EGR yet the truck still has the same issue. Any Ideas????

He could have carbon buildup on the injectors or in the throtle body. That will make it do that as well, you can use a can of brake cleaner to clean it out.