SG-II gauge/trip discrepancy
 

Purchased a new ScanGaugeII that arrived with firmware 4.13. Correctly entered engine displacement and tank size in setup. Signal mode is VPW. Followed procedure in product manual for first fill-up. In GAUGE mode, all parameters appear to be normal with the exception of MPG. MPG (not AVG) fluctuates wildly, even when driving on Interstate highway at steady speed of 60 mph. Minor fluctuations I would expect, but gauge jumps from 5 mpg to 80 mpg and everywhere in between, all while driving at steady speed for distance of 40 miles. It never stabilizes.

However, in TRIP mode, CURRENT MPG is stable and as expected, with only minor fluctuations for acceleration/deceleration and stop/go.

Phoned LL and they said it is normal for MPG to fluctuate because it is an instantaneous reading. Understood, but why so wildly, and why is CURRENT MPG in trip mode stable and as expected with no major fluctuations. Is this not instantaneous also? Why the discrepancy between "gauge" mpg and "current trip" mpg?

LL suggests that it will correct itself after 2 or 3 more fill-ups, but I am skeptical.

I use an Edge Insight and it does the same thing. The instantaneous MPG readout shows exactly what the car experiences in fine graduations. All the slight imperfections of the road, any declines and inclines, any wind, any steering corrections, just about anything can cause the car to fuel differently due to the constant variations that affect the car's progress. You can see the same thing with a vacuum gauge, especially those with 270-degree sweep dials.

As far as the other MPG values, they include an averaging factor which naturally changes more slowly.

"MPG" is a realtime calculation updated every 2 seconds. It is an instantaneous reading of the MPG at that moment in time. Even a slight change in conditions can make it change a great deal. Don't use it for anything except entertainment.

"Current" readings are for the current "trip", which starts and stops at your command. A "trip" might be a tankful, or it might be your commute to work, or it might be a drive across the state. The "current" readings are averages and change much more slowly.

This is all in the users manual. If you did not get one they are available online.

ScanGaugeII User Manual : Linear Logic

Thanks for the information. I had previously reviewed the SG manual, but could find no indication that the "Current" reading was an average. As for the "MPG" instantaneous reading, the fluctuations were so extreme that I removed it and replaced it with a different parameter, using only "AVG" for tracking fuel mileage.

It's probably your cars.

In all cars I've had my SGII in, I've been able to hold my instantaneous MPG dead steady in steady driving.

However, just about every car I've tried (especially newer cars), there's some compatibility issue with SGII or UG or whatever. It seems as cars get more complex, on size fit all gauges are more difficult to program.

Fluctuating MPG could also mean a bad MAF sensor.

Oldtamiyaphile, thanks for this useful information. I had actually suspected the extreme fluctuations might be due to a bad sensor, but did not know which one could cause this. Was not even aware of MAF sensor until you mentioned it, but will now definitely check this out. Thanks again!