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The data above shows the wideband O2 sensor reading at 12.46:1 AFR while the Commanded AFR is 12.81. Should this error be corrected? Not necessarily; it's a minor range of error. Did we correct it? Yes. Why stop short of a great job, right? In this case, we adjusted the air flow rate in the 8,700 Hz range by subtracting roughly 8% (the 3% rich condition as well as the -5.5% LTFT) from the airflow values. This new calibration was uploaded to the ECM and the fuel trims were reset to 0. While we were at it, we also made one more change. The owner of this car will only be using this Z06 for club track days and car shows, so we leaned out the fuel PE settings just a tad to command 12.8:1 at peak torque and 13.0:1 in the upper engine speed ranges. This change is shown below:
Although the LTFT in the data provided was +18.1% to +19.7% at the MAF frequencies in question, you can certainly add 20% to trim higher than necessary. Given the choice (and you DO have one, as you are the Tuner!), you'd like to have the ECM subtract fuel rather than add when in Closed-Loop, for if that's the case, you know that the airflow modeling is on the generous side and, therefore, when in Open-Loop, you're less likely to run lean in any given circumstance. Once you've made this one simple change, you can modify the table in the 3,000 Hz range, then 4,000 Hz, then 4,500... You can see where we're going with this and why it's best to log data at each frequency step mentioned previously while letting the LTFT stabilize. After you've adjusted the MAF table throughout the frequency ranges you logged data for, you can look for any anomalies in the shape of the curve and "smooth" the curve if needed. Save this calibration file as a new one and then upload it to the vehicle and then run the car again and log the data. You should be pretty much spot on the money, but if not, simply modify the table some more in a similar manner and repeat the process until you're satisfied with the end result. In our case with this Z06, we had a slightly rich running condition...only +3% according to the wideband O2 reading. BUT, notice also that the LTFT is already subtracting 5.5% (as it reads -5.5 for both cylinder banks)...
Let's assume that you've logged a considerable bit of data while running at normal operating temperature and you see that the fuel trims are off by a considerable amount. Consider the data below:
MAF HZ - 2500 2600 2700 2800 2900
LTFT % - +18.3 +19.5 +19.7 +18.1 +17.4
In the above example, the ECM is always adding more fuel as the LTFT values are positive. To correct this, open up your calibration in the HP Tuners Editor software. When you open up the HP Tuners Editor software, select Engine, then Airflow, and then click the Airflow vs. Freq Low button as shown below. You will see the Low Frequency airflow table as shown below (along with the helpful description box that HP Tuners provides below. When you hover the cursor over a button, this description box displays the function of the table). Notice the red-highlighted areas in the illustration. You can see that we selected the airflow frequency ranges between 2,500-2,800 Hz and that we entered "1.2" in the g/sec box. With "1.2" entered, when we click the "X" (multiply by) button, the selected values in the table will be increased by 20%. If we clicked the "+" button, the selected values would have 1.2 added to them, and of course if we pressed "=", the selected values would be changed to 1.2 (Soooo not what we want to do). Once these changes are made (multiplying by 20%), you can use the "Interpolate" buttons (just beneath the "copy", "clipboard", =, + and X icons) to smooth the values beside those you've just changed. It's best to save the "smoothing" for last as you may have an entire table to adjust, so get this done first, then smooth the overall table if necessary.