Tridon Non Locking Fuel Cap - TFNL227 Fuel caps are designed to seal the fuel system of an automobile. A fuel system includes the gas tank and its connection components, such as the fuel pump, fuel lines and regulator / valve assemblies. Fuel caps are typically made from either metal or plastic designs. Metal fuel caps may be used for non vented (sealed) or open (vented) systems where a vacuum is formed inside the tank by operation of a fuel pump but is not used to regulate pressure. Plastic fuel caps are used in vented systems because they cannot handle high vacuum without deforming or cracking. Fuel caps are available in both locking and non-locking options in various styles including metal bayonet, plastic screw and push and turn designs.