Check this out:
http://news.yahoo.com/news?tmpl=story&u=/ap/20040528/ap_on_re_us/sulfur_in_gas

It happened to my '93 Sport last year. I dropped the tank and removed fuel
pump/ tank level sending unit assembly and replaced with a new one costing
about $110.00 at a local parts store. Turns out it was simply the float ball
which had developed a pinhole leak and sunk. I understand that  you can buy
float separately from Ford. Using my old trusty Haynes manual,  I replaced
whole assembly anyway as the truck was ten years old.  I did this myself and
it wasn't too difficult at all except I recommend an extra set of hands when
you are actually ready to drop tank and then put it back up after the
repairs. I also knew how many miles I could go on a full tank so there were
less than two gallons in the tank making it very light. Also if you replace
whole assembly vice simply the float, check/compare very carefully the angle
of OE float switch linkage. Carefully bend to match if different. I failed
to do this so now when I top off, gage reads right at full line rather than
to the right as when new. Hope this helps. Steve
GAB wrote in message ...