How do I eliminate components from consideration one by one?

Question

Code P0455.  I was getting a loose gas cap display on the control panel in my vehicle.  I made an adjustment to the gasket on the gas cap and that display went away.  But after several starts the service engine soon light came on.  I checked the OBD-II and got the P0455 code.  I think the adjustment to the gas cap took away that item as the source of the problem and forced the system to now display the service engine soon light, an indication that the true problem is elsewhere.  I drive at least 110 miles per day round trip to work and back.  I filled my tank up several times and A) I don't smell any gasoline and B) I'm getting about 26 MPG.  The fuel efficiency listed for this vehicle is 20 MPG city and 28 MPG Highway. I now need to eliminate potential causes one by one.

Answer 1

P0455 Evaporative emission system gross leak detected

This code is a known code with the Fusion, at the filler neck , near the fuel tank filler pipe at the bend is where a crack are commonly found. The best way to verify this is with a smoke test, if there is a common crack you may even be able to smell its location out