Abstract: A device for preventing entry of an unleaded gas fill nozzle in a diesel fuel filler neck comprising an elongate sleeve configured to be inserted inside the diesel fuel filler neck. The sleeve provides for a finger tight or friction pressure fit at its upper end against the internal wall surface of the diesel fuel filler neck. A gas nozzle blocking screen or grid for preventing the insertion of an unleaded gas fill nozzle into the sleeve is located at or near the upper end of the sleeve. The invention also includes a filler neck cap. The filler neck cap includes a removable weather-tight cap through which the nozzle is inserted. The cap preferably includes a key lock to prevent its removal by anyone other than an authorized person. The removable weather tight cap preferably is tethered to the filler neck cap.