i have a 2002 gmc 2500 sierra light duty 4 wd extended cab pick up with approx 125000 miles. the problem started when my garage replaced the fuel pump & inline filter. the trucks milage dropped from approx 13 -14 mile per gal to approx 7-8 miles per gal after the install of the new pump. the truck was running veryy poorly prior to the old pump being replaced (like it was running out of gas) after the pump was installed the truck has been running great, except for the milage. the mach. tested the fuel pressure of the old pump and it was very low so it was replaced and the fuel filter was also replaced. the garage has looked at the truck 2 times since to try and determine why the mileage is so poor. theres no smell of gas and no leaks have been found around the fuel tank.

any suggestions would be greatly appriciated, thanks much, seabarge

?? You say it's running great right now and the MPGs seem to drop in half! There's almost no exception to a vehicle running poorly would get worse mileage than when running well - no codes or other problems noted - yet?

With the history I would seriously be looking for some kind of leak that may only do it underway while it's not noticed, or perhaps some other unrelated factors: Any chance of cheating where you buy gas? Gas getting stolen? Anything different in the before and after driving habits?

I can only guess: Could brakes be dragging and you don't notice it or something weird that escapes notice right now. Try a different place to buy gas if in any doubt,

T
Tom Greenleaf - MetroWest Boston - USA
http://www.acsource.com/

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http://www.autoacsystems.com/tomgreenleaf/

Thanks Tom for the advice. the garage is going to check for gas leaks while the truck is being driven and also the to make sure the brakes are working properly. at this point everything else is checking out good. i did read that a falty mass air flow sensor could cause a drop in gas mpg or at least hide the reason for it. when i went to and auto parts store to look into the cost for a fuel pump i was told that there are at least 5 different pumps for my truck, im wondering if the proper pump was installed and if not could that cause the mpg to drop.

The trick is that you said it's running great?? If the fuel pump was wrong or MAF sensor I wouldn't think it would run so well or could it be that it's just running so much better that you aren't noticing that it still isn't running its best?

Did tank come out for the fuel pump job - probably. There's plenty there that could leak - perhaps just splashing when driven or something that's just not making a smell while it's driven. Just gotta look around and if fuel pressure reads normal now and no codes it points to leak or the other possibilities of not getting the gallons stated at purchase or stolen fuel with some regularity is that's even possible,

T
Tom Greenleaf - MetroWest Boston - USA
http://www.acsource.com/

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http://www.autoacsystems.com/tomgreenleaf/

I would be looking at injector balance & volume tests as well as making sure that the O2(s) were functioning properly.
What I’m thinking is that the old fuel pump with its low pressure wasn’t able to feed the engine the same amount of fuel that the new pump can.
Which may have been covering up a potential injector issue or an O2 sensor that is constantly sending a lean signal to the computer?
With a lean signal, the computer will start to dump fuel in an attempt to make the O2(s) toggle to rich.
Yes most times there would be codes set but not if the O2(s) were still toggling enough to not meet the threshold to set a code?
Just thinking out loud and may be completely off track.

Dan.

Canadian "EH"

Good points Tom and Dan. Also there may be a possibility the new pump stirred up junk that has the regulator hung a little making it run just a little rich. Again, just a suggestion.




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