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In Reply to: Fuel delivery/OBC question.... posted by James '82 733i on December 08, 2001 at 20:18:48:
The fuel pump pushes fuel around to maintain a constant pressure, so fuel will pass through it (especially if it is on the feed side) at a constant rate whether the engine is idling or pushing you along at 130mph.
I don't know for sure, but the most likely way the OBC measures fuel usage is to count the length of electronic pulses sent to the injectors. Assuming the injectors are not clogged or broken, this would give the most accurate measure of fuel usage.
How does the figure match with your econometer on the instrument panel? If you drive along at a constant speed with your finger pressed down on the CONSUM. button (or safer, with a passengers finger!), is it a reasonably steady figure that is the same as the econometer? Or does it leap around?
I have been using the OBC to see the "novelty" of seeing my average mpg, it used to display around 14mpg, now its down to saying 7-8,mpg. But I drove about 300 miles to my brothers college, and I figured out I averaged about 14-16 mpg. So how does the OBC figure out the Average MPG? Also, I heard a noise back by the left rear wheel (where the filter is) that sounded as if it was clogged, but the filter was replaced about 9k miles ago, so what's happening to make the noise and make the OBC think I'm gettin 8mpg when I'm getting about 14-16mpg?
I guessed that the OBC figured out the average mpg by how hard/long the fuel pump is working, so if its clogged it would still get good mpg, but the OBC whould figure it wasnt becasue it was working so hard. Is that how it works or is it totaly different?
Any help would be good...Thanks,
~James
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