Very strange smell. Stumped

[Grumpyjoewv]

06 W9 ISX

I’m totally stumped. I’ve been getting an exhaust type of smell in the cab. Fairly strong. We’ve replaced the entire exhaust from the turbo to the stacks except for the muffler. New Y pipe, all new flex pipe, clamps, etc. Had two minor leaks at the manifold and after a quick re-torque, no more leak. There are no leaks anywhere. Here’s the first weird part. It only happens when the outside temp goes below about 35 degrees. The even stranger part is, it gets worse after the truck is parked and the engine is shut off. It finally gets so bad that I have to roll the windows down and open the sleeper door and grab the dog and get outta the truck. After an hour or two there’s no more fumes. What am I missing?