If it isn't the tank cap then I am thinking that you are thinking about the evap canister, as in dropping the old one and installing a new one. Reason my mind drifted in that direction is that I have an evap code showing on my dash as well. That means a new canister install on the spring to do list for my S-10 and I plan to include a new evap module as well. Both items are already in the bin and await warmer weather so I can bring the truck in and do the deed in relative comfort.
Nick
Nick