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  1. #1

    Default MPG decrease, what could be the cause?

    Over the last week I have noticed a drop in my MPG. My last road trip I was able to get 22 MPH on the freeway out of my 05 yukon xl. This trip I only got 17.9 mpg. I drove the same route, same speed, with the same weight. Tires are inflated to the same PSI. I have not added any mods to the truck since the last trip. I checked the plugs and they are new. The wires are new. I cleaned the MAF sensor and throttle body. There is not a changeable fuel filter. It idles and runs smooth. I can't complain to much as I a still getting 16mpg in the city, which is well over the epa rated 13 mpg. But, I still want my highway mpg back. Today I will connect my programmer and run the monitor to see what my O2 sensors are reading and if anything looks out of the ordinary with those. Any ideas about what would cause an instant decrease would be appreciated.

    I checked O2 sensor readings with my monitor while I drove. Both banks had the same reading. I did notice that the throttle percentage never went to 0, like my 02 did. It was always at 11% even with my foot off the gas and with the truck off. It does not have a tps like my 02 did. The hole were it was on my 02 is plugged on this truck.
    Last edited by Pikey; 10-03-2012 at 11:14 AM.

    1995 Silverado 4x4
    6" BDS Suspension Lift-3" Body Lift-Add A Leaf in rear -Trailmaster SSV Shocks-Duel Steering Stabilizer Kit -AirAid Cold air intake-
    4.56 Gears with Detroit Auburn Locker-Pro-Comp Traction Bars with duel shocks-Aluminum Skid Plate Kit-38.5" x 16.5" Mickey Thompson Baja Claws-Constant Dropping fuel gauge

    2005 Yukon XL Jet Power Programmer, Bilstein Shocks, Bilstein rear springs, Helwig Anti-sway bars, EGR Window Visors, EGR Hood Shield, Denali Headlights, Headlight harness upgrade, GE NightHawk Bulbs, White Night Rear lighting system, Russell Braided SS brake lines, PowerStop Brake pads, PowerStop cross drilled and Slotted Rotors,
    2002 Silverado ext cab 2wd (Sold)
    2003 Yukon XL (Totaled)

  2. #2


    So if your positive you don't see a filter on the drivers side of the frame rail then you only have to change the one in the fuel tank. But before you go that route check your injectors. Same gas and station?
    2001 Burban 4x 5.3L

  3. #3


    Quote Originally Posted by My4xburbanrocks View Post
    Same gas and station?
    I had this same question.

  4. #4


    Yes, I filled up at the same station I always do and refilled it at the same station at my destination that I always do.

    - - - Updated - - -

    I just talked to the manager at the Gas station I filled up at. She said that last thursday they switched to the "winter Mix" gas. I saw the tanker leaving as I pulled in. I don't know if that has something to do with it.

  5. #5


    Also you can put that injector cleaner in your tank and it should fix it within a week. thats what my dad did with his to pickups that were running a little ****ty. And it worked good.
    1994 GMC Yukon, 2 door, Victory Red
    Garage Link:

  6. #6


    Quote Originally Posted by Pikey View Post
    I just talked to the manager at the Gas station I filled up at. She said that last thursday they switched to the "winter Mix" gas. I saw the tanker leaving as I pulled in. I don't know if that has something to do with it.
    I don't know the mix differences. I do know that I get 1-1.5MPG (depending on how I drive) more on the winter mix than I do on the summer mix. However, I'm also usually not running the AC in the winter, so I don't think it's just an issue of the mix...

  7. #7
    Sr. Mechanic
    Join Date
    Jun 2011
    Tuscaloosa, AL


    Here's what I found on the mix gas

    Cars using summer-blend gasoline spew fewer harmful emissions and get slightly better fuel economy than those using winter-blend gas during the summer months.
    The difference between conventional summer- and winter-blend gasoline has to do with the Reid Vapor Pressure of the fuel. RVP relates to the volatility of a gasoline. The more volatile a gasoline, the more likely it will evaporate as the temperatures rises; evaporated gasoline contributes to unhealthy ozone and smog levels. Summer gasoline has a low RVP and is less likely to evaporate when compared to the high RVP winter grade. The Environmental Protection Agency says conventional summer-blend gasoline contains 1.7 percent more energy than winter-blend gas, which contributes to the summer blend’s slightly better gas mileage.
    The Energy Information Administration says the switch between the two fuels happens twice a year, once in the fall (winter blend) and again in the spring (summer blend). Summer-blend gasoline is typically more expensive to produce than the winter blend, and it won’t affect vehicle performance or the durability of the engine and fuel system, according to the EPA.
    In many large cities as well as California and New England, the EPA requires the use of reformulated summer- and winter-blend gasolines. These RFGs contain oxygenates that lower RVP and other toxic chemicals even further than conventional gasoline.
    2004 GMC Sierra SLT 6.0L 4x4
    UWS Low Profile toolbox 10% tint front and over stock back tint
    AirAid MIT and K&N airfilter
    BlackBear Performance

  8. #8


    Wow, good info, Sierra. I think that answers Pikey's question, outright. It also tells me that my AC use has a stupidly large impact on my fuel consumption and that it's the likely culprit for MPG declines in the summer -- not the mix.

    I really appreciate you taking the time to dig that up and share it. Have some rep!

  9. #9
    Sr. Mechanic
    Join Date
    Jun 2011
    Tuscaloosa, AL


    No problem at all, I was actually curious about that too cause I have never heard of different mixes. Haha thanks for the rep points!
    Last edited by sierra11; 10-03-2012 at 10:03 PM.

  10. #10


    The 4 mpg discrepancy is not just from the change in blends...

    The blends are for the different climates that we live in to prevent issues with fuel based on outside temps and also the laws on pollution levels allowed in your state.

    Your truck would have been tuned at the factory for 87 octane most likely being a small block and some big blocks require 91, 92, 93 depending what part of the country you live in.

    I'm not saying it's an octane issue more likely just dirt injectors and a couple of tanks with injector cleaner will probably fix it.

    But I do know a guy who is a chemical engineer that went through the actual process of testing fuel from different stations for there octane rating and discovered that on 2 occasions the fuel he pumped was not at the actual octane value it should have been.

    Thus affecting his tune and mpg.

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