Method of validating a diagnostic purge valve leak detection test

1. A method of evaluating the integrity of a leak detection test for a purge valve of a fuel system in a vehicle, said method comprising the steps of:

determining when vehicle soak conditions meet first predetermined criteria conducive to fuel vapor phase changes in the fuel tank;

determining when a vehicle maneuver meets second predetermined criteria indicative of the capability of the maneuver to initiate fuel slosh in the fuel tank thereby establishing a trigger event;

determining, after the trigger event, a maximum slope of fuel tank vacuum increase;

producing a slope ratio as a function of the maximum vacuum increase slope and a reference vacuum slope;

invalidating the purge valve leak detection test when the slope ratio exceeds a predetermined threshold.

2. The method of claim 1 wherein the first predetermined criteria includes a predetermined temperature drop, said step of determining when vehicle soak conditions meet first predetermined criteria includes the substeps of:

determining a first temperature parameter indicative of a minimum liquid fuel temperature during a first cycle;

determining a second temperature parameter indicative of a current liquid fuel temperature at the beginning of a second cycle subsequent to the first cycle;

calculating a difference between the first and second temperature parameters;

determining whether the calculated difference exceeds the predetermined temperature drop.

3. The method of claim 2 wherein said first temperature parameter determining step includes the substep of:

selecting a proxy from the group comprising an ambient air temperature, an intake air temperature (IAT), a fuel tank vapor space temperature, and a fuel tank liquid temperature;

monitoring, during the first cycle, a value of the proxy and determining the minimum value thereof;

assigning the determined minimum value of the proxy to the first temperature parameter.

4. The method of claim 3 wherein the proxy comprises the IAT.

5. The method of claim 3 wherein said assigning step is performed when a duration of an engine-run portion of the first cycle exceeds a predetermined minimum time period.

6. The method of claim 1 further comprising the step of determining an ambient soak time between an end of a first cycle and a beginning of a second cycle, wherein said step of determining when vehicle soak conditions meet first predetermined criteria includes the substeps of:

comparing the ambient soak time and a preselected maximum time period;

determining that the first predetermined criteria are not met when the ambient soak time is greater than the preselected maximum time period.

7. The method of claim 2 wherein said step of determining the second temperature parameter includes the substeps of:

measuring a start-up intake air temperature (IAT) value for the second cycle;

measuring a start-up engine coolant temperature value for the second cycle;

selecting the second temperature parameter from the lower of the start-up IAT value and the start-up engine coolant temperature value at the beginning of the second cycle.

8. The method of claim 1 wherein said step of determining when a vehicle maneuver meets second predetermined criteria includes the substeps of:

measuring a vehicle speed;

establishing the trigger event when the vehicle speed exceeds a speed threshold.

9. The method of claim 1 wherein said step of determining when a vehicle maneuver meets second predetermined criteria includes the substeps of:

measuring a vehicle acceleration;

establishing the trigger event when the vehicle acceleration exceeds an acceleration threshold.

10. The method of claim 1 wherein said step of determining when a vehicle maneuver meets second predetermined criteria includes the substeps of:

measuring a vehicle speed and determining when the speed exceeds a first speed threshold and thereafter determining when the speed declines to below a second speed threshold to thereby establish the trigger event.

11. The method of claim 1 wherein said step of determining when a vehicle maneuver meets second predetermined criteria includes the substeps of:

measuring a vehicle acceleration rate and determining when the acceleration rate exceeds an acceleration threshold;

after said acceleration measuring step, measuring a vehicle deceleration rate and determining when the deceleration exceeds a deceleration threshold to thereby establish the trigger event.

12. The method of claim 1 wherein said step of determining the maximum vacuum increase slope includes the substeps of:

monitoring a vacuum level in the vapor space of the fuel tank after the trigger event and for a remainder of the purge valve leak detection test;

calculating a plurality of slope values for the monitored vacuum level each taken over a respective preselected time interval; and

determining one slope value of the plurality of slope values that has the largest value.

13. The method of claim 1 wherein said step of producing a slope ratio as a function of the maximum vacuum increase slope and a reference vacuum slope includes the substeps of:

selecting the reference vacuum slope from the group comprising (i) a pre-slosh event vacuum level slope and (ii) a predetermined vacuum slope calculated at least as a function of a fuel tank vapor space or fuel fill level; and

dividing the maximum vacuum increase slope by the reference vacuum slope.

14. The method of claim 1 wherein the step of invalidating the purge valve leak detection test when the slope ratio exceeds a predetermined threshold includes the substep of:

discarding the results of the purge valve leak detection test.