I get this on my 2005 CTS-V - it's basically done it as long as I've owned it (bought it used in late '07), but it seems more pronounced now. I think what I've decided is that the cabin vent (which the Factory Service Manual calls the "pressure release valve") must be allowing some leakage in. The valve is located back in the trunk on the back side of the driver's rear wheel well. The valve is there to allow the pressure in the cabin to equalize due to temperature changes or use of the HVAC system, and as far as I know it's basically just a flapper valve. The FSM makes it look pretty easy to replace, just snaps in and out of position.
Here's my train of thought... I don't know if yours in the same, but in my case I only notice the smell when I come to a stop. Once I'm in motion the exhaust smell goes away, because the exhaust is being left behind, whereas while the car is stationary the exhaust can swirl around and get sucked into the cabin through a faulty pressure relief valve (which is just a couple/few feet away from the exhaust outlet). It happens even when I'm at a stop light or sign by myself, so I'm not sucking in someone else's exhaust. The HVAC intake is on the cowl at the base of the windshield on the passenger side (as it is on most cars); if the car was sucking in its own exhaust there, then I would also smell it in the summer when the windows are down. Since I don't, it's not coming in there.
Not sure when I'll get a chance to actually investigate my hypothesis, what with the cold weather and my driveway being encrusted in snow.
Here's my train of thought... I don't know if yours in the same, but in my case I only notice the smell when I come to a stop. Once I'm in motion the exhaust smell goes away, because the exhaust is being left behind, whereas while the car is stationary the exhaust can swirl around and get sucked into the cabin through a faulty pressure relief valve (which is just a couple/few feet away from the exhaust outlet). It happens even when I'm at a stop light or sign by myself, so I'm not sucking in someone else's exhaust. The HVAC intake is on the cowl at the base of the windshield on the passenger side (as it is on most cars); if the car was sucking in its own exhaust there, then I would also smell it in the summer when the windows are down. Since I don't, it's not coming in there.
Not sure when I'll get a chance to actually investigate my hypothesis, what with the cold weather and my driveway being encrusted in snow.
