[mdlug-discuss] OBD fuel monitor fine tuning

[Robert Meier]

Drew,

> No.
> However, I have devised a funnel which lets me get the fuel level in
> the main tank up to a consistent mark,

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

> 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>

> ... I ... try to avoid buying ethanol-blended fuels. And
> this because I don't get enough of a price break to make up for the reduced
> BTU content per gallon which would result.

I've observed the same.

> That much I already have in the Python code to OBDGauge.

> I am, however, confused about the directions of Rich and Lean in regards
> to the sign of the values.
> Both the OBDGauge User Guide and the Wiki page state that negative values
> for the fuel trim numbers (ie, raw values less than 128) indicate a Rich
> mixture.
> However, When I first start the engine,
> the values that I get for the short term fuel trims start
> out very strongly *positive* (raw values much more than 128), and
> gradually work their way to stoich; and I was under the impression
> that engines tended to run rich when first started. I just started my
> engine to double check this, and both short term fuel trims read over
> 40/128 to positive. (I display these numbers in 128ths instead of
> calculating "percent".)

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% .

My long term fuel trim has been rising slowly (becoming more positive,
actually less negative) since the catalytic converter was replaced.
The measured fuel consumption during the last three tanks has
decreased as the long term fuel trim values have become less negative.

As I believe that rising oxygen sensor voltage implies more oxygen,
and the engine control module should richen the fuel mixture,
and set fuel trim raw values less than 128.

Reporting,
-- 
Robert Meier

"To translate it into UNIX system administration terms (Randy's
fundamental metaphor for just about everything), the post-modern,
politically correct atheists were like people who had suddenly found
themselves in charge of a big and unfathomably complex computer system
(viz. society) with no documentation or instruction of any kind, and so
whose only way to keep the thing running was to invent and enforce
certain rules with a kind of neo-Puritanic rigor, because they were at
a loss to deal with any deviations from what they saw as the norm.
Whereas people who were wired into a church were like UNIX system
administrators who, while they might not understand everything, at least
had some documentation, som FAQs and How-tos and README files, providing
some guidance on what to do when things got out of whack.  They were, in
other words, capable of displaying adaptability."
      -- Neal Stephenson, Cryptonomicon, p.585