depending on if its an after market fuel cell, then the cap doesn't seal or vent like your DOT gas cap.
also there is no provisions for the emissions system equipment to plug in and work.
if you make a fuel cell, you need it to be exactly like the factory tank, and have to be able to prove the emissions system is fully working.
Edited: 10/3/2010 6:57:17 PM by Rusty Shackleford
I know this from first hand experience. Custom fuel cells will not pass emissions UNLESS you have a stock/factory-equivalent gas cap. California smog tests require a venting gascap. Then there is the evap canister blah blah blah blah. Best bet for Cali-smog is to get it done at a "not-so-reputable" shop.