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Old 12-09-2013, 11:33 PM   #1
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2013 Focus Buggy MPG Algorithm?

Howdy folks. I bought my 2013 Focus SE new in June and have noticed some buggy behavior with the mpg estimates. I previously owned a 2002 Focus and still own a 2009 Fusion, however neither have real time mpg estimation for me to compare to.

First, I have on two occasions caught the mpg ticking up slowly while the car is stopped at a stop light. It went from approximately 34.0mpg to 35.0mpg before I had to begin driving, at which point it fixed itself. The second time it was at 36mpg and ticked all the way up to 42mpg before I had to go. It was going up by 0.1mpg every few seconds. Had kind of a twilight zone moment!

I tried and failed to get a video of it on my cell phone. Damn things just aren't setup for quickly getting something like this captured and my concern was dismissed at the dealership as just being "how it works, it is only an estimate."

Ticking up when not moving tells me either the odomoter is broken, and the car thinks it is moving when it is not, or the algorithm has an integration error somewhere. Is this normal behavior? Other engineers believe the fuel is measured directly out of the injectors so as to be quite accurate and reliable. In both cases it occurred about 20 miles into a new tank of gas. So if they are trying to read a float or ultrasonic sensor, I can see where it might cause a little delay. But why would such sensor readings increase over a hundred seconds or more?

My second problem is the consistency of my mpg. On a full tank I start out getting 38mpg driving in the city. Every quarter tank the mpg drops about 2mpg, until it ultimately settles around 32-33mpg by the time I am at 1/10th of a tank. This is entirely repeatable, and kind of confusing. In reality I am seeing about 31mpg when estimated at the pump.

I'm only somewhat familiar with fuel economy variables, but is this to be expected? As someone who designs sensor electronics for many industries, I somehow expected mpg estimation to be more accurate.

Thanks for any insight you can provide me with!

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Old 12-10-2013, 03:16 AM   #2
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The fuel consumed side of things is based off the calculated fuel pump flow. Nothing in the car measures the amount of fuel used. There is simply an algorithm based on averaged pump duty cycle vs pressure vs time.
The distance side is based on the VSS.

Being the pump is PWM, the calculation isn't always dead on accurate. It should end up averaging out though (all things do with enough time, right?).

I have seen the complaint before and IIRC someone mentioned the remedy was a dealer reflash ... Which I'd guess just knocks the sensitivity of the fuel end of things down (PID priority).
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Old 12-10-2013, 05:25 AM   #3
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What I have noticed is if the MPG read out is reset while driving/accelerating that it will climb at a stop. Normally it will just drop after filling up when stopped.
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