Instant and Average Fuel Mileage Readings

[emongoose]

My instant and average fuel mileage readings on the dash used to be accurate. I've had a couple fuel system warrant repairs done, and now the readings are substantially lower. I've had a couple 400 mile road trips the last couple of weeks, and on level hwy, @ 75mph, the instant mileage reads 20-21mpg, instead of what usually read 25-27. I cleared the average reading this morning, and after a 240 mile drive, the average read 21.6mpg... but the actual based on fill up and miles driven is 27mpg.

If it always appeared to be wrong, then I'd just blame the Germans for not figuring it out, but it used to work just fine.

Does anyone know how mileage metering works? What sensors are used to guage fuel usage. Is there anything "actual" (like metering actual fuel flow?) or is it all extrapolated from throttle position, speed, etc?

I'm just trying to figure out what got hosed up.

[Tom K.]

Although my 1er's odometer is about 1.5% pessimistic, the OBC avg. mpg is accurate to within 1%. If it were my car, I'd carefully record the indicated avg. mpg and actual mpg for about 4 tanksful. If there is still a significant (5% or more) error, I'd take it back to the dealer to sort out.

Tom

[Joey_T]

You can manually correct the fuel consuption read out by adding the correction factor (which you calculate based on your calculated consumption vs. computer read out) into the secret menu 20.00.

OBC Secret menu

Instructions to access the hidden OBC functions:
1. Hold down Trip Reset button while inserting ignition key / Press and Hold down Trip Reset button >10 seconds when idle.
2. Right after – if you wait 2 seconds – menu 01.00 appears, where you can check your cars VIN number (you will need it to unlock all hidden functions). At this point only menues 01 and 02 are unlocked, others are still locked.
3. You can also skip waiting in step 2 (in case you already know VIN and you wish to proceed) and immediately use Trip Reset button to select function 19.00, where you can unlock all the other features. Indicator first shows LOCK ON.
4. Wait for display to show "CODE 00", then enter the sum of last 5 digits from your cars VIN number with Trip Reset button.
5. Other hidden functions are now unlocked and display starts with function 01.00.
6. To move between sub-menus press Trip Reset.
7. To change menus first depress Trip Reset for 2 seconds to appear »_ _«, then several times to select desired function.
8. Secret menu remains unlocked until you turn ignition off.

[emongoose]

Interesting...

Would a software update have affected the mileage calculator? It used to be quite accurate, but since the recent warranty work, it is now calculating approximately 20% below actual.

[Joey_T]

Hmm... I don't think that the software update could be the reason, but I could be wrong. In any case I suggest that between next two fill ups you reset consumption and mileage read out and calculate consumption by hand. Then divide this number with the consumption read out and multiply it by 1000 and you have a correction factor that you need to enter into the menu 20.00. If you're not comfortable doing this just ask the service guys to enter your calculated factor for you...

[emongoose]

Looks like I'll try to input the correction factor myself.... the dealer's service advisor has never even heard of the hidden or secret OBC menu.

[emongoose]

Originally Posted by Joey_T

In any case I suggest that between next two fill ups you reset consumption and mileage read out and calculate consumption by hand. Then divide this number with the consumption read out and multiply it by 1000 and you have a correction factor that you need to enter into the menu 20.00. If you're not comfortable doing this just ask the service guys to enter your calculated factor for you...

Ok Joey_T... I got all the way through the hidden menu, unlocked all the menus and navigated to menu 20.00. From there I could not figure out how to input my correction factor. There are 3 sub-menus... 20.01, 20.02 and 20.03, each of which have German text/abbreviations which mean something. Each starts with "Korr" which I imagine is a German abbreviation for Correction... but there was no place I found to input a factor. I needed to create a factor to increase the calculation 22%.

Underneth the abbreviation are numbers that continually to change in the display... no idea what those mean.

Anyway... although I feel like I adjusted nothing... I didn't knowingly input an adjustment factor... the instant mpg does now read substantially higher... maybe even too high. On the way home from work, on a flat road at 76mph... my instant mpg read around 33-34mph.

I'm not sure if that's right, but more importantly, I don't know what I actually did within menu 20.00 to adjust the readings.

Can you provide a little more insight? And now do have have to obtain a new average mpg... calculate it again actual usage, determine a new factor, and make new adjustments in menu 20.00?

[Joey_T]

This is a copy/paste from other forum (http://www.m3post.com/forums/showthread.php?t=111105):


20.0 Correction factor:

Shown consumption x 1000 / user defined consumption

Push the trip reset button to begin the correction. Numbers count up from 0-9, when correct number shows
push trip reset button.

20.1 Sets 10ths. for consumption factor

20.2 Sets 100ths. and 1000ths. for consumption factor

20.3 Saves the new consumption factor and shows new value

If your consumption is shown in mpg, then to calculate the correction factor you calculate it as the bold text above suggests (in my previous post I wrote it the other way around, which is true if your displayed read out is in L/100 kms - didn't know this before, but did some more research).

To enter the factor into the OBC, get into Test 20 and press reset. This will start the KVBR value count down. It will count down to 750, then start over at 1250. In other words value 1000 is a default number and you can adjust it + or - 25 % (in range from 750 to 1250). When the count down displays your correction number (for instance 1150) push trip reset button and you are set.

Since you have previously calculated your factor IMO no need to do it again. Just insert that number and for the next two or three tanks check if it is accurate. If not calculate it again and enter small adjustments.

Hope this helps


P.S. and you have some other cool features in those menus, check this thread: http://e90.wetpaint.com/page/BC+hidden+menus

For instance you can display your actual speed in digital form in menu 08.00 and the speed as it is shown on the indicator in menu 08.01.

[emongoose]

ok... That clarifies the "rolling" number display below the sub-menu titles on 20.01-20.03. Now I think I can make sense of the process.

But I do have one "uh oh" question. Now that I've been poking around the 20.00 menu, and obviously adjusted the factor without knowing what I was doing, I"m trying to understand the "fix" I'll have to use for the correction factor.

So using the formula Shown consumption x 1000 / user defined consumption I would get 21.6x1000/27=800. That actually seems backwards to me... I would consider a number below 1000 to be a factor reducing the mpg reading on the dash, and one greater than 1000 to be a factor increasing the mpg reading... but so be it. It's all just math... and depends how the engineers built the formulas to use the "Correction Factor".

So now that I've inadvertently made a factor change the shown consumption will now be different, which if it is still off from actual observed mpg, means that a new factor will have to be calculated, and input one time into the 20.00 menu. Meaning that if you input the incorrect factor, you can't just change it again without obtaining a new "shown consumption" to calculate a new factor... in otherwords... the factor is always cumulative.

Example: Assume now that my shown consumption is 30mph. So the formula would calculate as 30x1000/27=1111. That would be a new factor based on the now current shown consumption of 30mpg with the same actual mpg of 27.

I hope your eyes have not gone crossed from this explanation, but is this correct? If so, 1) I have a lot of adjusting to do... and 2) this sounds like you can really chase your tail, and because it takes a while to get a new shown consumption, this is an extended process.

So then I question what the menu 21.00 "Reset" does... would that reset the correction factor to factory spec? If so 1) that could put me back to where the reading was when I bought the car, or 2) I might get me closer for the correction factor to start. I must have played in menu 20.00 5 or 6 times... and if those adjustment factors are cumulative, I am significantly off.

[emongoose]

Ok... Update.


Did some in-car testing. The fuel mileage correction factor IS NOT cumulative, it is a simple factor applied to the calucated mileage to improve accuracy. That is foretunate for me since I had been fiddling with it some much... a cumulative adjustment would have been a big hassle.

When you are on menu 20.00... it will tell you what your current correction factor is. A 1000 is no factor. Anything below 1000 will increase the displayed mpg (both instant and average) from what the computer is calculating, and conversely for factors above 1000.

The first time I went to menu 20.00, my factor was set at 1142, which decreased my calculated fuel mileage approximately 20% from actual. I'm not sure if a program update during recent fuel related warranty work did that, or if the tech did something. Anyway, after trial and error to gain an understanding of how the correction factor functions. Returning the factor back to 1000 got the calculated mileage back pretty close. I have settled on a factor of 950 for now, and after another longer trip I should be able to get the caluculated mileage really close.

Anyway, now I know how to do it. And while I don't know precisely what caused my factor to be set to 1142, this should be something that other owners should keep an eye out for when there are programming updates or perhaps other work done on their car. Or if you think that your calculated mileage is slightly off (and that bugs you) then you have an avenue to go in and make adjustments.

To clarify the process...

Once you reach menu 20.00, after a couple seconds, the display with show your current factor. Pressing the trip reset button once will move you to sub-menu 20.01, which enables you to change the "lowest" digit in the factor (1000). This digit will count up from its current value, cycling through numbers 0-9 several times. When it reaches the value you want, press the reset button again. This will set the "ones" digit, and move you to sub-menu 20.02, which allows you to set the "tens" digit (1000). This digit will now count up through numbers 0-9 several times. Again, when it reaches the value you want press the reset button again. This will set the "tens" digit, move you to sub-menu 20.03, and now the "hundreds/thousands" digit (1000) will begin counting up from 7 to 12 several times. Same process as before will set this value.

To "save" your setting, press and hold the trip reset button 2+ seconds until the display clears and returns to 20.__.

At that point the new correction factor is saved. Remove your key fob, put key fob back in, start the car, and go test your results.

Another point to note. If after you set the correction factor to a value other than "1000", to further refine the accuracy of the calculated mileage, you will need to use the current correction factor as the multiplier to the calcuated mileage to determine the revised correction factor.

So the formula would be Shown Consumption x Current Correction Factor / User Measured Consumption = Revised Correction Factor.

[Joey_T]

I'm glad that you've worked it out The moderators could make "a sticky" of your detailed explanation on how to adjust it. Or even better you could open a new thread (under DIY) and copy your text into it. I think more people will see it there.

As far as a reset in the menu 21.00 goes - I wouldn't do it...

[E91 Hoarder]

Hey Folks,

Back to the OP's question, how does the fuel metering works in our cars?

Here is my problem: no matter driving style, the avg mpg stays constant at 6mpg. Even after reset, it immediately goes back to 6mpg and won't change. I did the miles/gal manual calculation, and the actual is around 21mpg.

I don't think the correction factor is a solution for my case.

2006 N52B30 325i, MT.

Any clues?