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Old 04-22-2011, 01:59 PM   #1 (permalink)
UFO
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Join Date: Sep 2010
Location: Denver, CO
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Colorado - '17 Chevrolet Colorado 4x4 LT
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SGII calibration on TDI (auto transmission)

OK, my SGII misled me on my last tank of fuel. I changed my driving technique to pulse and glide specifically to test fuel consumption. Before I changed how I was driving, the SGII was spot on with fuel used, I did not need to make a fuel adjustment over the last two tanks.

I used LOD and tank average, and SGII reported more than 3mpg more than the previous tank, same fuel, same commute. But I filled the tank last night, and my mpg was actually 3 mpg LESS than my last tank.

I think I understand why my actual mileage dropped, as I was accelerating to a higher speed than usual to coast down, and my average speed on the tank may have been higher (I saved the new fill-up before I thought to look at average speed).

But why is the SGII so sensitive to driving style? Has anyone had this issue with a TDI, where the calibration is good for one type of driving, and more than 10%-15% off for a different type?

Either the problem is the fuel consumption calculation at idle is too low, or the fuel consumption under acceleration is calculated too high. My previous calibration must have been accurate only for the balance of idle and acceleration I was doing previously, and when I changed, the "calibration" no longer applied.

Here is my thought on the most likely culprit: The engine displacement input to the SGII. I thought I knew what the mileage average should be, and I purposely entered a lower displacement than actual, 1.4 liters instead of 1.9. If this input to the SGII is how it calculates the idle fuel consumption, it would be too low. Maybe that's it, but I'd like to hear more input.

Thanks.

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