Abstract: A vent-on-demand fuel sump and vehicle fuel system having such a fuel sump are provided. The fuel sump may include a pressurized vessel and at least two sensors configured to detect a level of fuel within the vessel. A valve coupled to the vessel may be configured to release air and/or fuel vapor to the atmosphere. The fuel sump may also include a programmable electronic controller configured to modulate the valve between a closed position and an open position based on signals received from the sensors corresponding to the fuel level. The valve may be configured to remain in the closed position until the fuel level drops below a predetermined level and the controller sends a signal to open the valve to release air and/or fuel vapor from the vessel into the atmosphere. The vehicle fuel system having such a fuel sump may include a fuel container and an engine having an intake.