System and methods for evaporative emissions leak testing

A method for a fuel system, comprising:

• during a first condition, including a vehicle-off condition, where the vehicle-off condition includes a vehicle-off soak duration greater than a threshold,
• responsive to a first absolute fuel tank pressure being less than a threshold, maintaining a fuel tank sealed from atmosphere and from a fuel vapor canister for a threshold duration based on ambient temperature and/or time of day;
• indicating degradation of the fuel tank responsive to a second absolute fuel tank pressure being less than the threshold;
• responsive to the first absolute fuel tank pressure being greater than the threshold, coupling the fuel tank to the fuel vapor canister by actuating a fuel tank isolation valve positioned between the fuel tank and the fuel vapor canister while maintaining the fuel vapor canister sealed from an intake manifold of an engine and from atmosphere;
• and
• indicating degradation based on a change in pressure at the fuel vapor canister.

A system for a hybrid-electric vehicle, comprising:

• a fuel tank coupled to a fuel vapor canister via a fuel tank isolation valve;
• an engine intake coupled to the fuel vapor canister via a canister purge valve;
• a canister vent coupling the fuel vapor canister to atmosphere via a canister vent valve;
• a fuel tank pressure sensor coupled to the fuel tank;
• a canister vent pressure sensor coupled within the canister vent;
• and
• a controller configured with instructions stored in non-transitory memory, that when executed, cause the controller to:
• during a first engine-off condition, open the fuel tank isolation valve while maintaining the canister purge valve and canister vent valve closed, and indicate degradation based on a change in pressure at the canister vent pressure sensor;
• and
• during a second engine-off condition, open the canister purge valve while maintaining the canister vent valve and fuel tank isolation valve closed, and indicate degradation based on a change in pressure at the fuel vapor canister.