If tightening the gas cap didn’t work, borrow a known good one and try that. Many vehicles on the road today use the same style. If that doesn’t work, sometimes cleaning the gasket around the gas cap or wiping Vaseline on it can make for a better seal. After that, the next step is a visual inspection of the system. We always start at the gas tank and work our way outward. Look for disconnected or dry-rotted hoses, loose hose clamps, and cracks in plastic fuel system parts. Sometimes removing plastic trim and using a flashlight can save a ton of time, as seen on this TJ. Jeep used a lot of hard lines, then switched to rubber lines and back to hard lines again. Pay special attention to those lines, as they seem to crack faster than the longer runs of rubber.