Individual cylinder fuel control systems and methods for oxygen sensor degradation

1. A system comprising:

an offset determination module that determines an offset value based on an average response time of an exhaust gas oxygen (EGO) sensor located upstream of a catalyst in an exhaust system,

wherein the offset value relates one of a predetermined number of cylinder imbalance values to a cylinder of an engine; and

a correction determination module that determines a fueling correction for the cylinder based on the offset value and one of the predetermined number of cylinder imbalance values.

2. The system of claim 1 further comprising a final EQR module that determines a final EQR request based on a base EQR request and the fueling correction and that controls fuel to the cylinder based on the final EQR request.

3. The system of claim 1 wherein the offset determination module determines the offset value further based on an engine load.

4. The system of claim 1 further comprising:

a first response time module that determines a first response time of the EGO sensor to a rich to lean transition;

a second response time module that determines a second response time of the EGO sensor to a lean to rich transition; and

a third response time module that determines the average response time based on the first and second response times.

5. The system of claim 4 wherein the third response time module determines the average response time as an average of the first and second response times.

6. The system of claim 4 wherein the first response time module determines the first response time further based on a first average of a first predetermined number of previous rich to lean transitions, and

wherein the second response time module determines the second response time further based on a second average of a second predetermined number of previous lean to rich transitions.

7. The system of claim 1 further comprising:

a threshold determination module that determines a variance threshold based on an engine load; and

a re-synchronization module that selectively triggers a re-synchronization of the offset value with one of a predetermined number of new cylinder imbalance values based on the variance threshold.

8. The system of claim 7 further comprising a variance determination module that determines a variance value based on the cylinder imbalance values,

wherein the threshold determination module determines the variance threshold further based on the average response time, and

wherein the re-synchronization module triggers the re-synchronization when the variance value is greater than the variance threshold.

9. The system of claim 7 further comprising a variance determination module that determines a first variance value based on the cylinder imbalance values and that determines an adjusted variance value based on the first variance value and the average response time,

wherein the re-synchronization triggering module triggers the re-synchronization when the adjusted variance value is greater than the variance threshold.

10. The system of claim 7 wherein the offset determination module, in response to the triggering of the re-synchronization, determines the offset value using one of a function and a mapping that relates the average response time to the offset value.

11. A method comprising:

determining an offset value based on an average response time of an exhaust gas oxygen (EGO) sensor located upstream of a catalyst in an exhaust system,

wherein the offset value relates one of a predetermined number of cylinder imbalance values to a cylinder of an engine; and

determining a fueling correction for the cylinder based on the offset value and one of the predetermined number of cylinder imbalance values.

12. The method of claim 11 further comprising:

determining a final EQR request based on a base EQR request and the fueling correction; and

controlling fuel to the cylinder based on the final EQR request.

13. The method of claim 11 further comprising determining the offset value further based on an engine load.

14. The method of claim 11 further comprising:

determining a first response time of the EGO sensor to a rich to lean transition;

determining a second response time of the EGO sensor to a lean to rich transition; and

determining the average response time based on the first and second response times.

15. The method of claim 14 further comprising determining the average response time as an average of the first and second response times.

16. The method of claim 14 further comprising:

determining the first response time further based on a first average of a first predetermined number of previous rich to lean transitions; and

determining the second response time further based on a second average of a second predetermined number of previous lean to rich transitions.

17. The method of claim 11 further comprising:

determining a variance threshold based on an engine load; and

selectively triggering a re-synchronization of the offset value with one of a predetermined number of new cylinder imbalance values based on the variance threshold.

18. The method of claim 17 further comprising:

determining a variance value based on the cylinder imbalance values;

determining the variance threshold further based on the average response time; and

triggering the re-synchronization when the variance value is greater than the variance threshold.

19. The method of claim 17 further comprising:

determining a first variance value based on the cylinder imbalance values;

determining an adjusted variance value based on the first variance value and the average response time; and

triggering the re-synchronization when the adjusted variance value is greater than the variance threshold.

20. The method of claim 17 further comprising, in response to the triggering of the re-synchronization, determining the offset value using one of a function and a mapping that relates the average response time to the offset value.