Poor Gas Mileage and Messed up vents?? - Truck Forums

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Old 09-19-2010, 05:50 AM
GMCSierraFan's Avatar
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Default Poor Gas Mileage and Messed up vents??

98 GMC Sierra 2500 4x4 5.7 Auto. 12.2mpg. I drive like an old lady and couldn't possibly be more conservative with my pedal. Is there anything I can replace? I just bought the truck so most of the maintenance items have not been replaced yet. Ideas?

Also, when I have either the heat or the a/c on, when I try and crank the fan, it sounds like it's really putting out the air, but its really just kind of pissing out of all the vents. I didn't realize it was coming out all the vents until my windshield iced up a little one day from the air conditioning. Anyways, I was told there was a little door that needed to be freed up.. Anyone have a diagram or instructions for how to do this?

Thanks for all your replies,
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Old 09-19-2010, 08:32 AM
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Get down on the drivers side and look towards the middle of the dash where the A/C plenum is. There is a little motor there that switches the blend door. Take the motor out and see if the gear is cracked inside. With the motor removed you can turn the gear on the housing by hand to switch the air flow to see if the door itself is seized up. From my experience the gear in the motor cracks more frequently. Also pay attention how the little metal arm is attached, it can be a little tricky. Good luck!
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Old 09-19-2010, 09:51 AM
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Getting 12 mpg with your engine and truck is what I hear is about normal especially if you do much stop-and-go driving. I average about 13 mpg per tank with my 5.3/1500 where I live and average about 20 mpg on the highway.

I'd think about what things control fuel flow, ignition, and the sensors in the closed loop system to make sure everything is functioning right.

1. Clean your MAF.
2. Have the O2 sensors been changed within the last 100k miles?
3. Check the fuel pressure.
4. Check the fuel injectors with an injector balance test.
5. Run some Seafoam through the engine and tank.
6. Clean the throttle body.
7. Check, clean, or replace the spark plugs.
8. How old are the spark plug wires?

I have a Diablo Sport Predator that I swear improved my average tank and highway mpg by about 2 mpg. Getting a custom tune may help also. My mileage improved and I also have more power to go with it.

Others on the forum may have other ideas. I hope these help.
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Old 09-26-2010, 06:02 PM
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Well I took a look at the door today and discovered that while the selector that decides whether I have air at my feet or at my face works, no matter what setting the thing is on, air comes out of HALF of the defroster. It comes out of the driver side half only and there is no setting that will allow it to come out the passenger side OR stop coming out at all.

Any more input? The gear didn't look to be cracked
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Old 09-27-2010, 12:02 AM
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Kevin Im having the same problem with my 98 Tahoe I just bought. If I switch to blow to my feet, it blows good, but not much when set through the vents. I gotta look at it tomorrow and see what I can come up with. Oh and my 5.7 has been gettin bout 20 MPG average.
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