"Marsh Monster" <snottyman.DeleteThis@webtv.net> wrote in message
news:25037-41BD4BC0-194@storefull-3253.bay.webtv.net...
..
..
WARNING....
very long
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"Eightupman"
wrote in message
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If you live in an area of the country that "oxygenates" the fuel, you
may notice an decrease in mileage as well. It happened to me every
winter in NY, NJ when I lived up that way.....may seem like an
"overnight problem"
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On Dec 12, 2004, 1:51am
nomail.DeleteThis@spam.not (el Diablo)
worte:
Well I may just have to buy into the oxygenation of fuel theory.
I've filled up twice since this posting and both times I'm back up to
14.6 MPG and 14.4 MPG. And for the winter weather this is about right
for the truck. I do remember getting gas at a station that I rarely ever
go to about three weeks ago. I usually get my gas from a United Dairy
Farmers store, they have Mobile fuel and I've always had good luck with
that.
So I guess that I need to check the mileage a few more times and maybe
that's all it was.
Brian
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Devil Dude,
I would suggest checking the mpg a couple
more times before I became overly concerned
with the symptom.
as fer the code reader and other mentionings
in this thread.........
Random Thoughts::
A code reader.......reads codes.
A good scanner will show you the data.
When there are no codes, a code reader is useless.
By observing the data values that the
computer is showing, you can determine
if the sensors and the outputs are operating
correctly, and more importantly if the computer
is telling your truck to be a gas hog. Regardless of codes existing or
not.
GM vehicals use "long term" and "short term"
logic to adjust fuel delivery, aka Fuel Trim.
The values of the two on a scanner should idealy be the same under
"perfect" conditions.
The values are represented by a percentage.
(the same= reeelly, reeelly close)
(pefect conditions= nonexistant)
A "Drive Cycle" is logged each time the
computer is turned on and the factory
programed "required" conditions have
been met for logging a "complete drive cycle".
Eng. temp, tranny temp, mileage, stops, takeoffs, etc...etc..etc..
(drive cycle= each time you run to the liquor store...unless the liquor
store is jest down the
street, then it's = to each time you go a little
further than that)
GM computer systems are constently upgrading its command parameters
based
on drive cycle demand and percieved enviroment and load conditions.
(it adds more gas, air, spark, over long periods of time....not jest
when it wakes up one morning
and yer teenager with the lead foot drives it for
a day..what's the chances he's been drive'n it
to school fer the last 3 or 4 weeks..hehe)
It takes "up to" 50 drive cycles for the computer
to make a drastic change in fuel, spark, and
air delivery because of percieved changes in
the way the trucks being asked to perform.
Long-Term fuel trim data would reflect this.
(up to 50= i'm not sure, but i'm perty shure)
(perty shure= handgrenade close)
GM computers will also make immediate
short-term changes based on percieved
conditions.
(short-term fuel trim)
Nominal long-term fuel trim is around 128%.
Short-term fuel trim should reflect close to
that. If....the two values are'nt close, then
the computer's trying to make them close,
and if it's possible to do so....the values will
fall back in line with each other after several
drive cycles, and all will be back close to
normal again.
Gas mileage is directly related to O2 sensor
performance.
A sluggish O2 sensor does not always set
a code. It can be detected in data on a
scanner. Can be, not always.
A downstream O2 sensor that's reading
the same as an upstream O2 sensor will
not always set a code.....but by law ....
it's suppose to.
(but then...it's against the law to smoke
Hawain Red-Bud tooooo)
Your truck has several temp sensors and
engine "load" sensors, and sensors for
detecting quanity of air being used.
Cold engines need more gas.
Your computer "percieves" values given
it by sensors. It doesn't know if the values
are true.....and does NOT always set a
code for sluggish or even bad sensors.
Your computer will dump more fuel when
it thinks that the engine needs it, once it
figures out that what it dumped was to much,
it will take a little bit away and things will kinda
of level off again.
Your symptom could be.....
a bad sensor
a change in climate
a change in fuel grade
a change in driver habit
a change in engine mechanical condition
or....
normal
Averaging MPG over several tanks of gas is
perty sound way to diagnose an overnight
change in MPG.
It'd be a shame to pay someone to figure out
what the problem wuz....bout the same time
the computer took care of it on it's on.
End of Random Thoughts.
~:~
MarshMonster
~once i had this irish setter someone gave me,
and he wouldn't eat.....so i changed his brand of
dog food.....he wouldn't eat....so i got him some
better brand stuff......he wouldn't eat.....so i got
him some top-dollar Donald Trump oughtta be
so lucky kinda dog food......he wouldn't eat....
so i carry him to the dog doctor...................
....................................................................
....................................................................
fer $85 the doc told me the dog wasn't eat'n
rite~
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Hey Marsh good to hear from ya,
Yeah I agree with what you're saying, the first couple tanks that I got low
mileage on I was ready to start changing something. I did check for codes
and there were none, I then drive a few more tanks and the mileage came up
to almost what I had been getting in the spring.
So I'm starting to speculate that it was either a fuel quality issue or with
what you've said a product of the computer or sensors.
Question, do you know where there are any specific articles on GM computer
operation and programming? I'm somewhat of an amateur Visual Basic
programmer and can program PLC computers so I have a great interest in that
anyway.
Thanks for the info Marsh.
el Diablo
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