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Hey Mikey,

It's all about the A/F Voltage vs A/F Table under Sensors -> AFM/Mass Air Flow.

You NEED to datalog a good fifteen to thirty minute drive in order to get an average short term fuel trim compensation for various 'cells' in the vehicle's tune.

After you datalog, import it, then go to advance graphs and click X-Y graph. Plot short term fuel trim vs AFM voltage and look at where the fuel trim is deviating from 0. This table indicates, generally, how you need to increase or decrease the G/S air mass in the airflow vs MAF voltage table.

For instance, if your short term fuel trim, on average, is 5% at 1.2 volts, then that means in closed loop (obviously), your ecu is having to add on average 5 % fuel to compensate for a lean condition at that particular voltage. This indicates to us that our MAF meter is not "tuned" or adjusted at the 1.2 volts interval in our MAF Voltage vs Air Mass table. So, we need to tell the ECU that at 1.2 volts, the MAF is actually seeing more air mass than it thinks it is. Adding 5% air flow (right click -> Increase by percentage) is an adjustment that should enable the ECU to fuel the engine appropriately after just reading the air mass without having to leverage the primary o2 sensor in closed loop to make the correction after the fact.

Does that make sense?

The same principal applies when you're tuning for open loop/wide open throttle. The target wide open throttle lambda/A/F table is just that, a target... If your AFM Voltage vs Air Mass calibration is incorrect at the specific voltage intervals that the ECU is seeing when the TPS %/MAP reading meets the WOT point, then your wide open throttle air fuel is going to be off. There is no use of the primary O2 sensor in order to make a change on the fly, like you would with closed loop operating.

The goal here is to pull up the target WOT lambda table, while looking at your datalog. Find your WOT point, usually a really high TPS percentage, then look at what the actual A/F under WOT table is showing you. If it's hitting a far leaner target than you're commanding, then you go back to the MAF Voltage Table vs Air Mass and add air mass to that voltage interval, again referencing your XY graph. Instead of using short term fuel trim vs AF Voltage, use A/F Ratio (or A/F Ratio Corrected) vs AF Voltage. The percentage deviation at those higher voltage points (which you would never see being used in closed loop) will guide how much you add or subtract from the air mass figures in the air mass vs voltage table.

Let me know if that makes sense - if you would benefit from screenshots I'm happy to provide them. Send over your datalog, and we can dive into it!

FYI I'm no tuning guru, I just have a foundational understanding of how this aspect works. If I've gotten any of this wrong, please let me know!
 
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