Drinking Gas

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All thoughts welcome.
My 1999 K1500 that has a 5.7 liter automatic transmission with a 3:42 rear ratio has just recently started liking gas more than it usually does. I've
dropped from an average of 15 mpg to 13 mpg. Just sort of happened over night. The truck has only 67,000 miles on it and other than the sudden gas usage seams to run fine. I'm going to borrow a friends code reader this coming week and see if there are any hints in there. As far as I know the plugs and wires are original, I replaced the cap and rotor at 55,000 miles. I looked at the plugs then and they looked good still. Air and fuel filter was changed within the last 10,000 miles and I looked at the air filter today and it looks good. So in the event that I don't get any codes from the computer can anyone offer some suggestions? Could one of the oxygen sensors be bad and not give a code? I guess that I could have a leaky fuel injector too?
Thanks Brian
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bad O2 sensor will do that
if you can get a 'real' scan tool, watch the O2 volts
each sensor should switch back and forth, above and below 0.45 volts
if one is stuck on the low side, it is telling the ECM that there is a lot of free O2 in the exhaust, and the ECM adds gas

there
I
give
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I assume by a 'real' scan tool you mean a good one? I'm not sure what my buddies got until I borrow it. I may have to get it read some where or buy a descent one. Any suggestions for a good code reader?
Thanks Brian

rear
I've
gas
original,
and
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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"

there
I
give
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That's interesting, I live in Cincinnati Ohio and we have just had the weather change to cooler weather in the last few weeks. But as far as the fuel being oxygenated, wouldn't that be to increase the burning efficiency?
Thanks Brian

rear
I've
gas
original,
and
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You would think so, but it has more to do with adding a bit more alcohol I think in order to bring polutants down in the winter months with the low weather ceilings and all. I honestly would have to do a bit more research on the scientifics of it.
But I do know that I used to get worse gas mileage for sure, and was very evident in shorter trips around town. Longer trips tended to be just slightly less, but I chalked it up to having to stop and go when there was a winter accident on the roads....

over
last
anyone
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I believe California has some of the worst gas in the country (due to EPA regs "Helping" then environment) Get about 12 in the summer, 10 in the winter. Its been doing this for as long as I have owned the truck.
I love my old truck though, 1971 F250, 6 MPG, winter summer, full empty, It didn't care.
Eightupman wrote:

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Scott M wrote:

plugs, O2 sensors, air cleaner, fan belt, dist cap and rotor, trans fluid, and diff fluid are all less then two years old (15,000 miles max) Thinking about a set of MSD wires... but how much would that really help?
Here is something else Im thinking about. http://www.hughesengines.com/heads/edelbrock_magnum.asp
and the stage one: http://www.hughesengines.com/general/intakes/magnum_intake.asp
Stage one: http://www.hughesengines.com/general/fuel_injection/magnum_throttle_bodies.asp
hmm, this looks interesting too. http://www.hughesengines.com/general/fuel_injection/knock_sensor.asp
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Air cleaner and tranny fluid sound like they are due. Fram air filters are just as good any anything, and so cheap I just do them w/ my oil change - change the oil filter every time too. Cheap insurance...
GMC Gremlin

http://www.hughesengines.com/general/fuel_injection/magnum_throttle_bodies.a sp
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I'm just curious to ask since you guys mentioned the strict epa regulations in California, if you buy a brand new Chevy with a 5.3 Liter in Cali, and then go to Michigan and buy the exact same vehicle, is there going to be any difference in the two, emission control wise? I was just wondering if you moved from another state to cali, would you have to get your vehicle an emission test? Its a good thing Mich doesn't require emission testing, I think half the population would fail.

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regulations
any
GM and many other manufacturers have a 50 state emission rating for a couple of year now. Usually it's stated right on the vehicles window sticker as standard equipment.
Brian
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(due
summer,
owned
on
whats
fuel?
will
need
67K
help?
http://www.hughesengines.com/general/fuel_injection/magnum_throttle_bodies.a
http://www.hughesengines.com/general/fuel_injection/knock_sensor.asp
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For the life of me I cant understand why any sane and sensible person would want to move to Ca. Close behind Ca. is Va.
Whitelightning
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Whitelightning wrote:

I really don't know, its getting really crowded here too. Oh, and how about a 2,000 square foot house for ohh... $600,000? a little to pricey? well, you can start out by renting a closet sized studio apartment for about $1000 a month.
Oh, and don't forget the high marks the kids are getting in our over crowded schools.
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Chevy Guy wrote:

it is Required in the Area's Surronding Detroit
OAKLAND, WAYNE to name a FEW
I

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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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. . WARNING.... very long
===== ===== "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" ======== ======== On Dec 12, 2004, 1:51am snipped-for-privacy@spam.not (elDiablo) 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 ======= ======= 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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wrote:

First thing that I would do is change the plugs and wires. You've had them for six years and that's way too long. A visual inspection won't cut it regarding the wires. Just know that those wires and plugs are cranking hi volts all the time and wear on them is more like metal fatigue than something you would see visually. I change my wires and plugs at least every year and a half or sooner. Also dump a can of fuel injector cleaner into your gas tank four or five times a year. Hope this helps.
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Mars wrote:

MPG will drop in cooler weather and they change the volatility of gas in winter months and they are likely switching over now and when they do the BTU/energy content of the fuel drops and you might have gotten some gasahol (10 or 15%) which has a lower energy content too.
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