Q: Will letting fuel sit in a gas tank for over a year damage it or begin to rust it?

Although it is dependent on storage conditions, ambient temperature and so forth, generally fuel that is older than 3 months should not be used in a motor vehicle. Fuel that is one year old is clearly unusable and will damage the fuel system, as you draw it into the engine, by plugging up components. Year old fuel almost definitely will not start the vehicle (i.e., will not burn because the volatiles have already evaporated). The fuel tank should be thoroughly drained and the inside visually inspected. If the tank has solid residues (gum and varnish), it is possible that the fuel pump pick up screen will be partially blocked. The fuel filter should be changed and the fuel injectors should be removed from the fuel rail and sent to a lab such as Mr. Injector for replacement of the tiny internal filter baskets (really tiny) in each injector and ultrasonic cleaning of each injector. If you want to avoid problems and minimize diagnostic headaches when you restart the vehicle the foregoing list is the absolute minimum required work. If you desire that any of these services be performed by a certified mechanic, please use YourMechanic’s services page to request and schedule service. If you have further questions or concerns as you decide what to do, do not hesitate to re-contact YourMechanic because we want you to make the most of your repair dollars and help you get the BEST possible results.