Correcting OBC fuel consumption through unlocked OBC....

[Scott H]

Well, after a couple thousand miles on the 535iT, I am seeing 19mpg consistently (80% highway, 20%city). My OBC shows me 14.7 to 15.3 depending on where I am at in the running average. This is roughly a 24%-28% error. A Chicago area independent swapped an M30 + auto in to a 1992 525iT at the same time I did my project. He estimated 24% error on his about a month ago......so we seem to be on the same page. I think the reason for his finding being lower is that he does a lot more city driving than I do. The fact that my engine is pretty much rebuilt while his is a 135k mile M30 might contribute as well.

According to OBC function 20, I should be able to put a correction factor in to make the OBC display correspond closer to my actual fuel consumption......see link.

http://home.iae.nl/users/bts/obc.htm

In my 11/88 build 535 I could never get this function to work....the value remained 1000 no matter what I tried. I'm attributing this to early software of the 535 and assume my 3/92 should be able to work.

Anyone done this successfully, or tried and failed?

According to the instructions, I should find success by either calculating:
19/15.3 * 1000 = 1,242
or the reverse
15.3/19 * 1000 = 805

The first option seems more likely the way my brain works......but then again my brain has definitely been known to stop working.....often.

Thanks!
Scott

[mholbrook]

Keep us all up to date on this. I need to change my readout too. My 92 535im shows 17 when I'm getting almost 19 mpg. I'm going to wait until I complete valve adjustment, new rubber plenums, new fuel injectors and perhaps a new FPR depending on how the DME likes the new injectors.

Thanks for bringing this up and for the website.

[JonE]

I put in a number lower than 1000, I think. OBC was saying 15mpg, but calculation told me it was about 17mpg. I had to trial and error a few times to get it right. I believe once you've entered your correction figure that you must hit the S/R button to "save" it. I then hit the mpg button to see what it says. Sometimes it seems to revert back to 1000 for some reason after filling with gas and resetting OBC.
Hope this helps,
JonE

[Scott H]

Over the last few tanks I have noticed the same things you have....

My OBC economy correction factor resets to 1000 every once in a while. Interesting. The calc is done as follows:

old OBC mpg reading/your actual mpg calc = CorrFactor

CorrFactor * current OBC correction factor (default is 1000) = new number to enter in to OBC

I put in a number lower than 1000, I think. OBC was saying 15mpg, but calculation told me it was about 17mpg. I had to trial and error a few times to get it right. I believe once you've entered your correction figure that you must hit the S/R button to "save" it. I then hit the mpg button to see what it says. Sometimes it seems to revert back to 1000 for some reason after filling with gas and resetting OBC.
Hope this helps,
JonE

[les741]

If you lock the OBC again after changing the correction factor it won't reset again, at least it worked for me.