I found this and thought I would repost it. My car goes in for service Thursday. Will update after it is repaired.

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If you can smell exhaust when your car is just sitting it could be a couple of different things depending on the situation.
1. If you smell exhaust after you start it when the engine is cold (early in the morning or after it has sat for a while) and after the engine has warmed up the smell goes away then it could be a cracked exhaust manifold. The reason it goes away when it is warm is because the heat causes thermal expansion of the material the manifold is made of and the crack temporarily seals itself up. When it cools the crack opens up again.
2. Exhaust gaskets could be weak or bad. This could leak more exhaust towards the front and you could smell it in the cab. This won't go away whether the car is warmed up or not.
3. Your exhaust pipe running from the engine to the muffler could have a small hole in it. Again thermal expansion could seal this up. If you look under the car and can see exhaust coming from anywhere but the end of the muffler when it's cold then this could be it.
With all of these an increase in exhaust volume (sound, not size) is noticeable. If your car is louder when you push down on the accelerator than it used to be then this could help narrow down the problem if you told an expert.