A fuel cap, also known as a gas cap, protects your fuel supply by sealing, and providing cover for your fuel tank. Fuel caps are designed to prevent fuel loss, and keep out contamination. Without a functioning fuel cap, gas will evaporate and reduce your fuel efficiency by 1-2%. If your fuel cap is missing, you could potentially introduce contaminants that can clog your fuel filter. One of the more common causes of an illuminated check engine light is a loose, or missing fuel cap. If you suspect your fuel cap needs to be replaced, or your check engine light is on, give M & J Complete Automotive Care in Modesto a call at 209-408-0922 .