HELP! 1997 E-420 P0455 EVAP Leak

[tfwaok]

The dealer replaced the fuel cap but the problem persists. First the flashing low fuel indicator then the check electronics and CEL light come on. I took it back to the dealer and they now suggest the gas tank is the problem and want to to have it replaced at a whopping $1800.

I have a data logger and can reset the CEL but it's annoying as heck. My gut tells me it's not the tank and if I add some duct tape to the cap and place it on the problem stays away longer.

How can I tell if it's the sensor that's wrong? or, how can I just eliminate this vac test and is that a bad thing?

HELP!

Tommy in New York!