I have a bad smell too. I was towing my boa home from Tennessee and I stopped at a light and the smell was so strong it made me sick to my stomach. It was 97 degrees out. I got out to investigate and found some fluid all over my wheel well, drivers side front. I limped into the GMC dealer in Knoxville who told me I blew the Vent hose on my crank case. I thought replacing the hose would clear up the smell figuring the oil had gotten on the engine or exhaust and would burn off. 2,000 miles late the smell is still bad. Threw a code for my DEF (Which I have a constant problem with and GMC refuses to help) and a code for Knock sensors. My new Service dealer says the smell may have come form the knock sensors. Still investigating., BYW truck has gone through two complete DEF systems, an exhaust filter and sensors and 3 Cat converters in less than 50K miles. What a heap of crap! And to make it even better. the GMX Customer service rep just told me that she spoke with the sdealer when I questioned that the smell may be coming from the Converter and she called me back to tell me the "You don't need no CADILAC Converter!" Yikes!!!! GMC 3500HD Denali Dually