I agree with Cees and Jerry - it's probably a fuel injector or the cold start valve allowing extra fuel to run into a cylinder after the car is turned off. It stays there until you start the car then gets burned, creating your fuel smell and white/gray cloud.
Only other thing it could be is engine coolant, but you would not get a fuel smell from this. From what I have read (no personal experience), when a head gasket goes bad and some engine coolant leaks into the cylinder it burns white or gray. I don't know how to test for this, maybe a compression test on the cylinders, a compression test on the coolant system, and check to see if you are loosing coolant. Just guesses.
Rodd
Powell, Ohio, USA
1966 230SL, Euro, Auto, Leather, both tops
1994 E420