More to share on tuning using S300.
Background: I was tuning my car for part throttle (cruising) and managed to have a stable 14.7 AFR reading through column 1 to 5. However after I shut down the engine for a while (couple of hours) and drove again, I got lean readings (greater than 14.7). I was expecting richer AFR than 14.7 rather leaner reading as the engine has cooled down.
I did a datalog and realised that the IAT readings were quite high which caused the ECU to reduce the duty cycle, resulting in a leaner AFR reading. This might be the IAT sensor being heated up by the hot intake manifold.
2 lessons learnt:
1) When tuning your engine, try to make sure the IAT and ECT values are the same for each run as it affects the AFR value greatly.
2) If possible, try to find a way to insulate the IAT sensor or minimise heat transfer from the Intake Manifold to get a more accurate reading.
Once u manage to tune your engine as desired (with constant ECT and IAT values), you can adjust the fuel compensation for IAT and ECT accordingly since the default value may not be the ideal compensation required.