. . WARNING.... very long
=3D=3D=3D=3D=3D =3D=3D=3D=3D=3D "Eightupman" wrote in message . 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" =3D=3D=3D=3D=3D=3D=3D=3D =3D=3D=3D=3D=3D=3D=3D=3D On Dec 12, 2004, 1:51am snipped-for-privacy@spam.not (el=A0Diablo) 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 =3D=3D=3D=3D=3D=3D=3D =3D=3D=3D=3D=3D=3D=3D 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=3D reeelly, reeelly close) (pefect conditions=3D 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=3D each time you run to the liquor store...unless the liquor store is jest down the street, then it's =3D 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=3D i'm not sure, but i'm perty shure) (perty shure=3D 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~