Engine Light (Emmisions)

I have had this light reset 4 times allready. It all started when I got gas and left the car running. So it was reset and then a 144 ERROR came up which is leak in the emissions which we thought was putting the gas in, then we thought it was the gas cap, so we replaced it with a 740i gas cap and that wasnt it so I bought a Brand New 528i one and that wasnt it. The mechanic says he took every single line and tested them and he took one part out and had it tested (I was there and seen him do this), H esaid the only other thing he can do is a Smoke Test. I am imagining this is going to cost a bit of money.
Has anyone had anything similar to this or does anyone have any suggestions before I dump $$$$ into this Smoke Test?
Please any Help will be Appreciated Greatly!!!!!
PS: I just passed NJ Inspection Yesterday and the light was Not Showing because he reset the code when he did the tests . The Reset normally lasts 2 days and then the light is back on again.
Thanx In Advance
KB528i