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Old 08-26-2012, 08:03 AM   #21
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Quote:
Originally Posted by mtnguy View Post
Yep….the Scan Gauge is due for a little adjustment.
I've only had mine long enough to make 1 adjustment. How many times hve you adjusted yours? I was hoping after 2 or 3 it wouldn't need anymore.
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Old 08-26-2012, 09:10 AM   #22
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This has gone way down the rabbit hole. Can anyone comment on the original question?
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Old 08-26-2012, 10:07 AM   #23
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Quote:
Originally Posted by Len & Cheri View Post
I've only had mine long enough to make 1 adjustment. How many times hve you adjusted yours? I was hoping after 2 or 3 it wouldn't need anymore.
I adjusted 1 time after the 1st couple of fill-ups, and 1 time a year or 2 ago after I started keeping a spreadsheet.
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Old 08-26-2012, 10:13 AM   #24
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Originally Posted by wincrasher View Post
This has gone way down the rabbit hole. Can anyone comment on the original question?
Threads have a way of doing that, I am myself am guilty of contributing to that problem...sorry about that.

I think once you mentioned "the computer says 23.2", that started the discussions on the accuracy of onboard computers. But it has been an interesting discussion.

Hopefully the thread will get back to your original questions.
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Old 08-26-2012, 10:16 AM   #25
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The ScanGauge worke great on my '05 5.4. It was always within a couple of tenths on the number of gallons it computed vs what went in the tank. Unfortunately, I can't seem to get it adjusted for the Ecoboost. I can get it zeroed in for non-towing but then it is way off when towing. Not sure why.

As for the truck's computation of fuel consumed, it is always within a few tenths of a gallon vs the actual; whether I'm towing or not. On a 36 gallon tank, the error is negligible.
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Old 08-26-2012, 12:45 PM   #26
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Originally Posted by Dave_Monica

The fuel pump maintains the required fuel pressure needed for the injectors. The injectors determine the flow, not the pump.

Dave
Well that makes sense I suppose... Last time I worked on engines was the days of carborators. So, constant pressure pumps feed a metering device..... Still the same. Fuel flow is known.....
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Old 08-26-2012, 01:04 PM   #27
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Nope, fuel flow is not known, but is calculated. In a gas engine, the fuel pump is capable of over 80 psi. This pressure goes to the fuel rail that feeds the injectors, but is controlled to around 40 psi at idle, and the rest of the fuel takes the return trip back to the tank. The pressure regulator is controlled by vacuum, and will allow an increase in pressure, under load, low vacuum, to allow more fuel to be fed by the injectors. The computer controls the timing of the injectors, thus knowing how long each one is open, and knows the fuel flow at given pressure. The problem is, the regulator has a tolerance of +/- maybe 5 psi for given vacuum. The injectors also have a tolerance. 20 lbs per hour may be 20.5 pound per hour. Depending on the vehicle, the accuracy of the injectors, and how well they match, (the computer assumes all match set), and the accuracy of the pressure regulator, determines how close the calculations are. Basically, the computer is dead on with the calculations, it is the tolerances in the equipment that cause the error. Still, without a true fuel flow meter, it is all in the math. Tank gauges are not accurate enough. Hope this helps, and gets back to the original post.
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Old 08-26-2012, 02:24 PM   #28
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On the newer trucks the Driver Information Center measurement of gallons used is VERY accurate. It actually measures the fuel going to the injectors. Therefore, as long as you don't reset the gallons used, from new, you can divide the miles on the odometer by the gallons used to get an ACCURATE mpg. On mine I have seen the same mileage as the OP but over the entire tank the best I've seen is 17.5 (combined - iPhone app)
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Old 08-26-2012, 09:53 PM   #29
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Yeah, like I said..... Measured/known... And of course it's considering tolerances of the system. I admittedly don't know what the tolerances are, but my Chevy reads good.

And I think this is along the topic of the thread.... Why did the op's reading get whacky?
Perhaps something squirrelly with something in the system at a particular speed... Such as a weird fluctuation in fuel pressure at a given speed .....
I would wonder, tail wind? Could be more likely.

When I bought my silverado a few years back, I plotted several data points at various speeds.... Indicated avg MPG at something like 35mph, 45,50, 55. 60, 65, 70, 80
All points on cruise control sets, and several points at each speed to average out terrain, headwinds, etc... Interesting bell curve.
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Old 08-26-2012, 10:48 PM   #30
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I suspect that at 80-85 mph, in an unloaded condition, the engine is at it's optimal efficiency, but that is only a guess. I know guys that get the same mileage no matter what they do. In my Suburban, the faster I go, the worse it gets. So go figure.

I've also found, that some times, I do get better mileage not using the cruise control, so it could be that. My "natural" driving style is pretty efficient I guess.
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