I looked at the GMC 6.6 Duramax Forum and read about other people having the same problem when the truck goes into a DEF regeneration cycle. There is mention of a TSB (Technical Service Bulletin) for this problem and parts to order but I haven't found it yet. The dealer Mataga Buick, Cadilac, GMC in Stockton, CA., did not want to read the information about this problem I was showing them on my cellphone. This is a leased truck, so if it doesn't get fixed soon I will contact the lease company about the problem. The new truck smell now smells like diesel fuel smell. I can't keep this truck with this problem.

Update from Dec 27, 2014: I finally got this problem fixed at Sanborn Chevrolet in Lodi, CA. They were aware of the bulletin about this problem & fixed the problem. I needed a way home because they needed to keep it overnight. I lived too far away for their shuttle service so I needed to get a rental car. I called General Motors Customer Service, explained to them what was happening, and they arranged a free rental car for me. Thank you GM. I haven't had the problem with diesel fumes in my cab. The new truck smell is still present. I love this truck. It pulls my 27' travel trailer with no effort & am getting 17-18 mpg on flat land, 15 mpg in the mountains. Exhaust break works great. This truck is quieter inside than my car.

Update from Dec 27, 2014: The problem has been fixed by another dealership (Sanborn Chevrolet in Lodi, CA.) that knew about the bulletin to fix this problem. I contacted the GM Customer Service Division about this issue & they arranged for me to get a free rental car so I could get home (I lived too far away for the dealership shuttle). The smoke problem hasn't occurred again with my mileage at approx. 5800 mi. This truck has been great to pull my 27' travel trailer w/big slide. This truck is quieter than my car inside. I get 15 mpg in the mountains & 18 mpg on flatland. The ride is also better than my car.