Jerry, In my XJS, about 4 years ago, I had the same problem, and it was worse when the tank was just filled. I cleaned out the boot totally, and wiped up the very tiny 'wet area' on the boot floor and On the pressure tank. I then put down tissue paper in the area just under the 'pressure tank' for the overnight. After several tries, I finally I traced the 'weep'/fumes to a pinhole on the left corner of the pressure tank. It seems the stamping really stretches the metal here when the tank was formed. I first tried some aviation ga tank putty, but it didn't hold after the weather turned cold. So, after I used up most of the tank, I plugged the hoses with bolts and let the tank evaporate. I then used brake cleaner and let the tank 'dry' for a week. I then used some marine Tex epoxy on the pinhole corner and let it set for another week. I haven't had any problems in four years.
Sherman D. Taffel, Pres.
Columbia, MD
NCJOC
'72 E Type Ser III Coupe
'76 XJ12C
'89 XJS Convert
'96 XJR