My car has a fuel line leak. Not sure what one is leaking.

Hello, Dave here, contributing to RepairPal and providing expert advice since 2010. Your vehicle indeed has a "return line". It would seem too me that if the fuel stops leaking once the vehicle is running that your leak is indeed from a return line. If the key was on with the engine off, the fuel pump is primed to provide fuel and under pressure the line would leak if it was a supply ine. Either way, you must repair this before operating the vehicle, is it is a fire hazard. Seek local guidance at a shop near you. https://repairpal.com/auto-repair-near-me Click the link or call us at 877-913-1554. We'll gladly find you a certified RepairPal shop in your area.