Method and system for fuel vapor control

A method, comprising:

• monitoring a fuel system including a fuel tank, a vacuum accumulator, a venturi, and a canister between the tank and an engine, including:
• during an engine-on condition, storing vacuum in the accumulator via flow through the venturi;
• and
• during a subsequent engine-off condition and while the canister is isolated from the tank:
• applying the stored vacuum on the canister, and indicating degradation based on a fuel system pressure change following vacuum application.

A method, comprising:

• monitoring a fuel system including a canister, a fuel tank, a vacuum accumulator, and a venturi, the canister coupled between the tank and an engine intake manifold, including:
• during an engine-on condition, storing vacuum in the accumulator via flow through the venturi;
• during a subsequent engine-off condition, applying the stored vacuum on the fuel tank before the canister during higher fuel tank pressures, and applying the stored vacuum on the canister before the tank during lower fuel tank pressures;
• and
• indicating degradation based on a fuel system pressure response to the vacuum applications.

A method, comprising:

• monitoring a fuel system including a fuel tank, a vacuum accumulator, a venturi, and a canister between the tank and an engine intake, the vacuum accumulator coupled to a purge line connecting the canister to the engine intake, including:
• during an engine-on condition, storing vacuum in the accumulator via flow through the venturi;
• and
• during a subsequent engine-off condition and while the canister is isolated from the tank:
• applying the stored vacuum on the canister, and indicating degradation based on a fuel system pressure change following vacuum application.