Method and system for fuel system diagnostics

1. A method for a vehicle, comprising:

responsive to a refueling request,

actuating an isolation valve to a second position to depressurize a fuel tank via a depressurization port of a canister;

during depressurization, selectively indicating degradation of the first valve based on a rate of pressure decay in the fuel tank;

after depressurization, actuating the first valve to a first position and initiating fueling; and

in response to each of the rate of pressure decay in the fuel tank during depressurization being lower than a decay threshold rate and one or more premature shut-offs during refueling, indicating degradation of a cross over valve (COV) of an evaporative leak check module (ELCM) housed in a vent line.

2. The method of claim 1 , wherein upon actuation of the first valve to the first position, the fuel tank is fluidically coupled to a load port of the canister leading to a first buffer in the canister, and wherein upon actuation of the first valve to the second position, the fuel tank is fluidically coupled to the depressurization port of the canister leading to a second buffer in the canister, the second buffer including a smaller absorbent area relative to the first buffer.

3. The method of claim 2 , wherein the depressurization port is positioned closer to a vent port of the canister than the load port, and wherein the load port is closer to a purge port of the canister than the depressurization port.

4. The method of claim 1 , wherein during depressurization of the fuel tank, a refueling lock is maintained in an engaged position, and the rate of pressure decay in the fuel tank is monitored via a fuel tank pressure sensor, and wherein initiating fueling includes disengaging the refueling lock allowing fuel to enter the fuel tank.

5. The method of claim 2 , wherein selectively indicating degradation of the first valve includes in response to the rate of pressure decay in the fuel tank during depressurization via the depressurization port being lower than a first threshold rate, actuating the first valve to the first position to depressurize the fuel tank via the load port of the canister.

6. The method of claim 1 , further comprising, in response to each of the rate of pressure decay in the fuel tank during depressurization being lower than the second threshold rate and a pressure in the fuel tank during refueling being higher than a threshold pressure, indicating the first valve to be stuck in the first position.

7. The method of claim 6 , further comprising, during refueling, in response to the pressure in the fuel tank being lower than the threshold pressure, indicating the first valve to be stuck in the second position, the method further comprising, in response to indication of the first valve being stuck in the second position, limiting an amount of fuel in the fuel tank to a threshold level, the threshold level lower than a maximum fill level of the fuel tank.

8. The method of claim 1 , wherein the actuating the first valve to the second position to depressurize the fuel tank via the depressurization port of the canister is in response to a lower than threshold load in the canister, the method further comprising, in response to a higher than threshold load in the canister, actuating the first valve to the first position to depressurize the fuel tank via the load port.

9. The method of claim 8 , further comprising, during depressurization of the fuel tank via the load port, in response to the rate of pressure decay in the fuel tank being lower than the first threshold, indicating blockage in the load port and actuating the first valve to the second position to depressurize the fuel tank via the depressurization port.

10. The method of claim 5 , further comprising, upon completion of refueling, actuating the first valve to the third, closed position and engaging the refueling lock.