Methods and systems for a pressureless fuel tank

A vehicle system, comprising:

• an evaporative emissions canister;
• a variable volume fuel tank, the variable volume fuel tank sealed except for only a fuel tank isolation valve port and a fuel filler pipe, the variable volume fuel tank further comprising:
• bellows;
• an atmospheric port coupling an internal volume of the bellows to atmosphere or to the evaporative emissions canister;
• and
• a bellows valve positioned between the atmospheric port, a conduit coupled to atmosphere, and a vapor line coupled to the evaporative emissions canister, wherein the bellows valve is a three-way valve;
• and
• a fuel tank isolation valve (FTIV) in fluid communication between the variable volume fuel tank and the evaporative emissions canister.

A system for a hybrid vehicle, comprising:

• a variable volume fuel tank;
• a fuel tank isolation valve configured to adjust coupling between the variable volume fuel tank and a canister;
• a bellows valve configured to adjust coupling between an interior volume of a bellows and one of the canister or atmosphere;
• and
• a controller comprising computer-readable instructions stored on non-transitory memory thereof that when executed enable the controller to:
• execute a leak test diagnostic in response to a key-off event, the leak test diagnostic comprising indicating a degradation of the fuel tank isolation valve in response to a fuel tank pressure decreasing;
• and
• diagnose a presence or an absence of a leak in the bellows in response to the fuel tank isolation valve not being degraded via adjusting the bellows valve to fluidly couple the interior volume of the bellows to the canister.