MPG clears on its own

[gthberg]

My average fuel economy has been clearing on its own when I turn the car off. When I restart, it's blanked out as if I manually did it. Doesn't happen all the time…maybe 50% of the time. Is there some kind of hub battery for the OBC?

Thanks

[Hoost]

Car's battery is probably on it's way out. My OBC was resetting itself every time I started the car (among other issues), replacing the battery fixed it.

[DrFerry]

Originally Posted by gthberg

My average fuel economy has been clearing on its own when I turn the car off. When I restart, it's blanked out as if I manually did it. Doesn't happen all the time…maybe 50% of the time. Is there some kind of hub battery for the OBC?

Thanks

When the resting battery reserve gets below a certain threshold (~12.6Volts) the rolling cyborg known as the E9X M3 - with its ~15 onboard ECUs - starts shutting down resources - such as the clock which starts running slow (usually the first sign of a failing battery). Then other gremlin codes start coming out of the woodwork such as "increased emissions" or worse like a C.E.L. Using a voltmeter measure the voltage across the battery terminals after the car has been "asleep" for about six hours or more. It should be ~13.3Volts for a battery in good health. I posted a photo of mine somewhere on these forums. To measure the voltage use the red positive terminal and grounding post under the hood. Here's a link to a "Best Battery" thread..
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https://www.m3post.com/forums/showthread.php?t=1737565
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https://www.m3post.com/forums/showpo...85&postcount=4
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[cloumeM]

Originally Posted by Hoost

Car's battery is probably on it's way out. My OBC was resetting itself every time I started the car (among other issues), replacing the battery fixed it.

Yes, this! I had this started on me recently (trip and clock resetting) and it turned out to be my battery needing replacement. No more issue after the replacement.

[gthberg]

Originally Posted by cloumeM

Yes, this! I had this started on me recently (trip and clock resetting) and it turned out to be my battery needing replacement. No more issue after the replacement.

Any other ideas? I hope it's not the battery, as it's less than a year old…

[cloumeM]

Originally Posted by gthberg

Any other ideas? I hope it's not the battery, as it's less than a year old…

Less than year old battery shouldn't give you this issue. However, just to be safe, you can test the battery using a tester from autozone or other car parts stores. I purchased a battery charger that has the tender and diagnostic features from advanced autoparts for something around $30. Upon hooking up the battery to charge, the device displayed a sign that indicated faulty battery. I believe my old battery was about 3 years old based on the date on the battery. So I replaced it, coded to new battery spec (used Protool), and have not had the issue since. I think it would be worth a try to get the battery tested. I believe one of these part stores can test your battery at no charge. If it is the battery, it might be a cheaper fix compared to other parts which may need to be replaced that is causing the battery to not properly charge. Good luck!

[Hoost]

Could be parasitic draw and/or the alternator not properly charging the battery.

[gthberg]

Bumping this thread for any other ideas? Battery and alternator were replaced last year. This problem seemed to disappear, then came back with a vengeance this week….it’s resetting every time I start the car.

As soon as it cranks, the MPG shows 99.9, then goes blank and resets.

Thanks