At 102K miles, I've begun to smell burned fuel through the vents of my 2009 Lucerne CXL. It's not a strong odor, just annoying and concerning. I smell it through the vents when I pull away after sitting at a stop light for a minute, or anytime the car sits and then I begin moving again. My dealer has checked the car stem to stern, replaced a bunch of gaskets and seals on the engine and even performed a major test of the exhaust system and found absolutely no leaks. Nothing they've done has reduced or stopped the smell one bit. Now that summer has arrived, I've noticed that it doesn't happen very much when the climate control is in the AC mode rather than the heat mode. The car is in excellent repair, having been dealer-maintained by the book since day one. Anyone else ever experience this, and if so, were you able to find the source?