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Old 04-22-2011, 01:59 PM   #1 (permalink)
UFO
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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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Old 04-22-2011, 07:02 PM   #2 (permalink)
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I've had the calibration be way off on my gasoline powered Focus when there is a dramatic change in temperature.
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Old 04-26-2011, 03:22 PM   #3 (permalink)
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One piece of data I have confirmed is the idle fuel consumption is more or less accurate at 0.1 gal/hr.

I have set the displacement back to 1.9 liters, and adjusted the fill quantity down. After I adjust for the current tank of fuel and try to keep the driving consistent, I will try again to push the envelope to see if the calibration is more flexible to driving style.

Otherwise, how can we use the damn thing to evaluate changes?
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Old 04-29-2011, 03:33 PM   #4 (permalink)
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Quote:
Originally Posted by UFO
Either the problem is the fuel consumption calculation at idle is too low, or the fuel consumption under acceleration is calculated too high
If your real tank was LOWER than your calculated one, it could be that the opposite the case - idle may be too high and acceleration calculated as too low ?

Do you have a link for the 0.1GPH idle figure ? I have a 1.9 TDI and mine is measuring 0.12-0.14 GPH - I have configured it as a 1.4 and adjusted 50% of the difference on each tank - my last one was 1.5% out. I have not yet had a 100% exact match but I accept anything under 3% as being inside a margin of error - things like temp when filling up, slope of the car, my ability to 'vent' enough and so on. Plus my tank to tank is not consistent driving - speed, temp and so on. It can't be as a tank for me lasts 3-5 weeks.

I mainly use the SG2 figure to monitor instant MPG - e.g. keeping my instant MPG above my average, as well as other figures such as coolant (grill blocked), MAP and so on.

As for P&G I didn't notice much of a difference in using vs not on my TDI - the difference was within my 3% difference and on one tank better and on one worse for P&G than a non-P&G tank. My tanks last too long to experiment with different speeds really, I could get better results if I did more miles.

My best result has been with cutting out idle whenever I can. I have, for example, switched one part of my route to actually go for a portion which is stop-start vs crawl - I can stop the engine in the stop part but not in the crawl. My last tank gained 8% on this change.

Mine is a manual but I would think that at a stop light you engine is not only idling but also pushing against the torque converter and so your idle GPH I would think is higher.

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