Update: Got the fault code read at Autozone (thanks, Boston). It read as an fault of the evaporative fuel system-major. Guy at Autozone said he had a similar problem with his GMC truck. Said the fuel cap itself may be faulty and not creating a proper seal around the filler opening, thus trigerring this fault. He says he can't get me the cap and to stick with OEM.

Does this sound right? Is it a DYI or should I take to the dealer? If dealer, anyone have an idea of cost?

Thanks again, X5World!