[mdlug-discuss] OBD fuel monitor fine tuning

[Drew]

At 02:47 AM 10/21/07, Robert Meier wrote:

>I fillup until the nozzle pressure switch turns off the pump.
>This seems to be repeatable within 1%.

     This might be feasible in my case, but by doing so I sacrifice
up to around four gallons of capacity. Besides which, a cup is
0.625% of 10 gallons, and 0.3125% of 20 gallons (nominal capacity
of main tank).


> > without the possibility of the hose
> > sucking back any of my fuel.
>
><IIRC>
>I don't believe the hose sucks any fuel from the car's tank.
>Fuel/emission regulations require that the nozzle detect the rise
>in pressure that results as the tank fills,
>and return the fuel remaining in the hose to the station tank.
>The fuel remaining in the hose did not reach the car's tank.
>The fuel remaining in the hose has already passed the
>volumemeter and been counted toward the sale price,
>but its return to the station tank does not rewind the count.
></IIRC>

     Well, it's not going to do that either. I keep the end of the hose
well away from the liquid, so that there will be no rise in pressure.
If it passes the volumemeter, the station does *not* get it back
when I'm pumping it.


>Observation of my car indicates that negative percentage
>(raw values less that 128) does indicate a rich mixture.
>
>During the last three weeks that I have been able to observe,
>my short term fuel trim output has resembled white noise.
>By eye, the short term fuel trim appears to move more negative
>when the oxygen sensor voltages rise.  The correlation appears to be
>about -10 - -20% .

     So, do engines run lean on startup, instead of rich? If so, then it should
be possible to improve fuel economy by removing the thermostat thus keeping
the engine cold longer. I still maintain that my OBD is giving me raw
values much greater than 128 with the engine cold, and closer to 128 once it
warms up. Could the signs be reversed in some models? Could it be a
defect in my engine module? (My OBD also lies about vehicle speed, though
the number that it gives *is* linearly proportional to actual speed.)

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- Drew.