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  #1  
Old 10-19-2021, 06:38 AM
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Fixing OBC Fuel Consumption

Hi all!


So I've been trying to find a solution to this problem for a while, but without much luck I thought I might turn the question over to the forum.



First of all - sorry about posting another fuel consumption thread, I see there are already a few around, though they didn't hold the answers I needed!



So I have a 2006 3.0d x5 - that is the M57D30TU, with the 6HP26 auto - and the fuel consumption is actually pretty decent. I think.

It's hard to nail down what is 'normal' for one of these cars (especially one with over 470k km), but I've been tracking my mostly suburban stop/start fuel consumption over the last ~7500 km and I'm getting 11.1L/100km on average. That number is what I've calculated manually from fuel fills.
Though I don't have the records, if memory serves, on the highway it drops to about 9L/100km.


Anyway - the problem isn't the true fuel consumption, but rather the OBC reported fuel consumption. The figures vary a little from one fill to the next, but the true fuel consumption is typically ~47% higher than the OBC figure, and I want to fix that.



I realise that there is an adjustment that can be done to correct the OBC, but I believe the adjustment envelope is +/-30% so this in itself wouldn't be enough.



Other threads have said that if the car has been tuned it can mess up the OBC fuel numbers, but the previous owner bought the car new, and knowing them pretty well I'm confident that they wouldn't have done anything like that. I asked them about the OBC numbers, and they told me that it used to be very accurate, but one day it quite suddenly changed, even though the true fuel consumption remained the same.



So not being super familiar with the way the E53 goes about calculating its numbers, I'm wondering if there is anyone who might know what could cause this, and where I might start to look for the fault!



Thanks for any advice!
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  #2  
Old 10-19-2021, 08:27 PM
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Welcome Andy,

I have the same car - with less distance traveled (~250,000km).

Although I know this isn't your question, your consumption figures seem about right - maybe a tad high but that may be down to tyres, load and your driving style. I get 10-11l/100km around town and on flat, straight highway runs can get below 7l/100km (best I've had is 6.7l/100kms - nearly 1,400kms on a tank!).

I'm not 100% certain how the OBC exactly computes the consumption data but it has to be fuel used over distance (obviously!). It would be worth having a look at few values in the "unlocked" BC (assuming you have a high cluster with the text display).

First to check is the reported momentary consumption in the BC - menu #4. 4.0 will show the momentary l/100km value and 4.1 will show the momentary l/h value. Check these when driving at a constant speed and see if they match the gauge on the dash and are even sane numbers.

Next you can check test #8 - momentary road speed and verify that is sane also.

I think menu #7.4 also shows vehicle speed - not sure if it momentary or averaged.

Let us know what you find...
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Old 10-19-2021, 08:40 PM
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Oh and then of course there is the "fiddle factor" you refereed to - menu #20

TEST NO. 20
BC consumption value correction factor. This test adjusts the correction factor for the consumption value displayed in the MID. The production line installed value is 1000. The value ranges from 750 to 1250.

To adjust the correction factor press the trip meter reset button once for a reduction of 1.

For each press of the reset button the value decreases by 1 until it reaches 750. After 750 the number will reset to 1250 and begin to count down again.

To accept the set correction factor press the CC button.

The consumption correction factor (VK) is calculated from the actual amount of consumed fuel (VBR IST) and the displayed value (VBR ANZ):

VK = (Actual MPG / Displayed MPG) X 1000.
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Old 10-20-2021, 08:52 AM
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Well I sure do feel dumb right now. Went back and looked at the consumption correction factor and it's not set to the default 1000, but rather 0779.



That's 221 under 1000, presumably what it was before it was changed to 0779.
Perhaps unsurprisingly, after doing a little math, 221 above the default 1000 is just about the right number.


Looks to me like at some point in the past it's gone in for a service and they've made an adjustment, but the made the adjustment in the wrong direction...


Now that the number is where it should be, I'll watch the trip computer over the next week or so and see how it plays out.



Can't believe I didn't notice this before... haha


PS - Are you the Wayne who made up the great little video on youtube regarding the thermotop remote heater modification?

Last edited by AndyE53; 10-20-2021 at 09:00 AM.
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Old 10-20-2021, 02:08 PM
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Glad you found what may be the smoking gun in this (fuel) murder mystery... please let us know how it turns out.

Yep, that's my idiot self on the Webasto video... seemed like a good idea at the time.
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Old 10-20-2021, 05:09 PM
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After one short trip - which is in no way conclusive - OBC figures are much closer to the mark. I'll keep an eye on it and dial it in over the next few weeks and update here along the way.

Since I saw that video a few years ago, that mod has been on my to do list haha!

Thanks for the help
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Old 10-29-2021, 11:12 AM
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So it's been a week, and about 400 km later, and I filled the car up to find that the reading is still a little bit out, but waaaaaaaaay better.
My commute to work and back (mix of suburban main roads and back roads) - after going over tyre pressures and such as suggested to remove anything that could be hurting my economy - cost me 10.0L/100km. Computer was saying 9.7 L/100km - so only a small difference there.
Then immediately went onto the freeway, about 130km non-stop at about 110km/h. I reset the trip computer at the start and filled up again straight away at the destination, and got 8.0L/100km - computer was saying 7.7 L/100km.
I'll keep tracking it and if it's continuously optimistic by that same 0.3 L/100km, I'll make another adjustment, but it seems a lot better.
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Old 10-29-2021, 11:33 AM
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Just an FYI...if you track the actual MPG & the OBC's MPG over a long period of time...you may find that they really aren't that far off even though they never may be the same at each fill up.


You can see this in the spreadsheet below. It shows the OBC & actual MPG calculation for every fill up in 2013 on my e53 X5. By the end of the year, the totals for each column was very close...there was only a difference of 0.37. After I saw this I stopped worrying about the difference because it was so slim.


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Old 10-29-2021, 08:58 PM
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It would be great to see the data that you managed to collect on your E53, but it looks like the spreadsheet didn't attach.

I've been doing the same thing though, tracking my fuel consumption since Feb, and I have found that there is some variability in the comparison between calculated and OBC consumption readings that isn't related to the calibration offset - this makes sense to me - things like how far you fill the tank up, how accurate the fuel bowser is, etc, that would play into the accuracy of the calculation.

The goal for me is to get the OBC reading right in the middle of the expected variation. If sometimes the reading is 0.2 L/100km above the true reading, and other times it's below, then that's okay, as long as it's roughly equal. If I can get it to that point, I'll be pretty happy. I expect that when I do, I'll find the same thing that you did, where the long term fuel economy calculation is pretty much spot on.

I realise that I'm being slightly over the top about it (sorry!), but working in instrument calibration has resulted in my being easily bothered by gauges that spit out the wrong numbers...
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Old 10-29-2021, 10:05 PM
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Quote:
Originally Posted by AndyE53 View Post

I realise that I'm being slightly over the top about it (sorry!), but working in instrument calibration has resulted in my being easily bothered by gauges that spit out the wrong numbers...
If you’re bothered by gauges that spit out wrong numbers,

You’re gonna hate your Temperature gauge…




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