Since I had the S300 installed and tuned for more than one month ago, I have been trying to fine tune the fuel. The main problem that I had was to enable closed loop operation after I have complete my open loop tuning. The AFR runs really lean (17.0) after I enable the closed loop operation even though AFR reading during open loop operations were fine.
After going through the Hondata forum, I found some solution.
1) As the car was usually driven in closed loop operations, the ECU would have Long Term fuel trim inputs which will affect the duty cycle. The solution is to reset the ECU before tuning.
2) The symptoms above which I describe apparently affects only some ECU, the work around is to set the short term fuel trim settings under the closed loop tab to 0 for both maximum and minimum short term adjustments and tune the car in closed loop operation instead of open loop operations. (remember set the values back to the original after completion of tuning)
3) Next is to adjust the Rate of change to the recommend setting by hondata as below:
This will minimise AFR fluctuations, thus improving drivability and milage. The settings that I used was 5 for all slow value and 99 for all Fast value.
Plse note: The tuning done on my car was based on the PLX wideband AFR. Plse do not attempt to tune the car without at least wideband AFR, however u may try altering the rate of change even without a wideband AFR.
Short term fuel trim is the amount of fuel adjustment made by the ECU depending on the AFR reading
Long term fuel trim is the amount of fuel adjustment made by the ECU based on the average Short Term fuel trim made over a period of time
Rate of Change is the rate of change of fuel trim made by the ECU.