System and method for diagnosing flow through a purge valve based on a fuel system pressure sensor

1. A system comprising:

a pressure sensor that generates a pressure signal indicating a pressure within a fuel system of an engine;

a pressure variation module that determines an amount of variation in the pressure signal over a period based on a running total of a difference between a present pressure reading and a previous pressure reading over the period; and

a purge flow diagnostic module that diagnoses a fault associated with flow through a purge valve of the fuel system when the pressure variation amount is less than a threshold.

2. The system of claim 1 wherein the pressure variation module determines the pressure variation amount based on a running total of an absolute difference between the present pressure reading and the previous pressure reading over the period.

3. The system of claim 1 wherein the purge flow diagnostic module determines the threshold based on a predetermined relationship between a flow restriction in the purge valve and the pressure variation amount.

4. The system of claim 1 wherein the purge flow diagnostic module adjusts the pressure variation amount based on an amount of boost provided to the engine during the period.

5. The system of claim 1 wherein the purge flow diagnostic module determines the threshold based on an amount of boost provided to the engine.

6. The system of claim 1 further comprising a vent valve control module that opens a vent valve of the fuel system during the period.

7. The system of claim 1 wherein the pressure sensor is located in at least one of a canister and a line extending between the canister and the purge valve.

8. The system of claim 1 further comprising a purge valve control module that maintains a duty cycle of the purge valve at a predetermined percentage during the period.

9. The system of claim 8 wherein the predetermined percentage is within a range from 25 percent to 75 percent.

10. A method comprising:

generating a pressure signal indicating a pressure within a fuel system of an engine;

determining an amount of variation in the pressure signal over a period based on a running total of a difference between a present pressure reading and a previous pressure reading over the period; and

diagnosing a fault associated with flow through a purge valve of the fuel system when the pressure variation amount is less than a threshold.

11. The method of claim 10 further comprising determining the pressure variation amount based on a running total of an absolute difference between the present pressure reading and the previous pressure reading over the period.

12. The method of claim 10 further comprising determining the threshold based on a predetermined relationship between a flow restriction in the purge valve and the pressure variation amount.

13. The method of claim 10 further comprising adjusting the pressure variation amount based on an amount of boost provided to the engine during the period.

14. The method of claim 10 further comprising determining the threshold based on an amount of boost provided to the engine.

15. The method of claim 10 further comprising opening a vent valve of the fuel system during the period.

16. The method of claim 10 further comprising generating the pressure signal using a pressure sensor that is located in at least one of a canister and a line extending between the canister and the purge valve.

17. The method of claim 10 further comprising maintaining a duty cycle of the purge valve at a predetermined percentage during the period.

18. The method of claim 17 wherein the predetermined percentage is within a range from 25 percent to 75 percent.