Fuel system leak check based on fuel reid vapor pressure

1. A method for an evaporative emissions leak test, comprising:

adjusting a pressure threshold based on a fuel volatility of a fuel contained in a fuel tank; and

performing the evaporative emissions leak test based on the adjusted pressure threshold, where the fuel volatility is determined by:

venting fuel vapor from the fuel tank to a fuel vapor canister; and

determining a hydrocarbon percentage of the vented fuel vapor with a hydrocarbon sensor coupled between the fuel tank and the fuel vapor canister.

2. The method of claim 1 , further comprising:

determining a fuel Reid Vapor Pressure based on the determined hydrocarbon percentage of the vented fuel vapor.

3. The method of claim 1 , where the evaporative emissions leak test is an engine-off natural vacuum test, and where performing the evaporative emissions leak test further includes indicating degradation based on comparing a fuel tank pressure to the adjusted pressure threshold.

4. The method of claim 3 , where the adjusted pressure threshold is a threshold for a fuel tank pressure rise portion of the engine-off natural vacuum test.

5. The method of claim 4 , further comprising:

responsive to the fuel tank pressure being less than the adjusted pressure threshold following the fuel tank pressure rise portion of the engine-off natural vacuum test, performing a vacuum portion of the engine-off natural vacuum test.

6. The method of claim 5 , further comprising:

responsive to a fuel tank vacuum being less than a vacuum threshold following the vacuum portion of the engine-off natural vacuum test, indicating an evaporative emissions system leak.

7. The method of claim 1 , where the evaporative emissions leak test is an evaporative leak check module based test.

8. The method of claim 7 , further comprising:

isolating an evaporative leak check module from the fuel tank;

activating a vacuum pump comprising the evaporative leak check module;

drawing a vacuum across a reference orifice; and

setting the pressure threshold based on the vacuum drawn across the reference orifice.

9. The method of claim 8 , further comprising:

coupling the evaporative leak check module to the fuel tank;

activating the vacuum pump; and

drawing a vacuum on the fuel tank.

10. The method of claim 9 , further comprising:

responsive to a fuel tank pressure being greater than the adjusted pressure threshold, indicating an evaporative emissions system leak.

11. A method for an evaporative emissions system leak test, comprising:

determining a reference vacuum threshold;

venting fuel vapor from a fuel tank;

determining a fuel Reid Vapor Pressure of the fuel vapor;

adjusting the reference vacuum threshold based on the fuel Reid Vapor Pressure;

drawing a vacuum on a fuel tank with an evaporative leak check module; and

indicating degradation of the evaporative emissions system based on the adjusted reference vacuum threshold.

12. The method of claim 11 , where determining the reference vacuum threshold further comprises:

isolating an evaporative leak check module from the fuel tank;

activating a vacuum pump comprising the evaporative leak check module;

drawing a vacuum across a reference orifice; and

determining a reference vacuum in the evaporative leak check module.

13. The method of claim 12 , where determining a fuel Reid Vapor Pressure of the fuel vapor further comprises:

venting fuel vapor from the fuel tank to a fuel vapor canister;

determining a hydrocarbon percentage of the fuel vapor with a hydrocarbon sensor coupled between the fuel tank and the fuel vapor canister; and

determining a fuel Reid Vapor Pressure based on the hydrocarbon percentage of the fuel vapor.

14. The method of claim 13 , where indicating degradation of the evaporative emissions system based on the adjusted reference vacuum threshold further comprises:

coupling the evaporative leak check module to the fuel tank;

activating the vacuum pump; and

determining a resulting vacuum in the fuel tank.

15. The method of claim 14 , further comprising:

indicating degradation of the evaporative emissions system if the resulting vacuum in the fuel tank is less than the adjusted reference vacuum threshold.

16. A method for an evaporative emissions system leak test, comprising:

responsive to an engine off condition, closing a canister vent valve;

determining a resulting pressure of a fuel tank;

determining a Reid Vapor Pressure of a fuel in the fuel tank;

determining a threshold pressure based on the fuel Reid Vapor Pressure; and

comparing the resulting pressure of the fuel tank to the threshold pressure.

17. The method of claim 16 , where determining a Reid Vapor Pressure of a fuel in the fuel tank further comprises:

opening the canister vent valve;

determining a hydrocarbon percentage of a fuel vapor in the fuel tank; and

determining the Reid Vapor Pressure of the fuel in the fuel tank based on the hydrocarbon percentage of the fuel vapor.

18. The method of claim 16 , further comprising:

responsive to the resulting pressure of the fuel tank being greater than the threshold pressure, indicating no degradation of the fuel tank.

19. The method of claim 18 , further comprising:

responsive to the resulting pressure of the fuel tank being less than the threshold pressure, closing the canister vent valve; and

comparing a resulting fuel tank vacuum to a vacuum threshold.

20. A method for an evaporative emissions leak test, comprising:

where the evaporative emissions leak test is an evaporative leak check module based test, isolating an evaporative leak check module from a fuel tank;

activating a vacuum pump comprising the evaporative leak check module;

drawing a vacuum across a reference orifice;

setting a pressure threshold based on the vacuum drawn across the reference orifice;

adjusting the pressure threshold based on a fuel volatility of a fuel contained in the fuel tank; and

performing the evaporative emissions leak test based on the adjusted pressure threshold.