Abstract: A cylindrical fuel cap having a first chamber filled with vapor absorbent material, a second chamber filled with air filtering material, and a one-way check valve for establishing communication between the atmosphere and the interior of the fuel cap is placed in a fuel inlet line of a fuel tank to trap fuel vapors in the tank preventing them from being emitted to pollute the atmosphere and to provide a fuel vapor reservoir for augmenting the fuel used for combustion in an internal combustion engine, oil burner, and the like, upon demand.