Fixing OBC Fuel Consumption

[AndyE53]

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!