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MPG decrease, what could be the cause?

Discussion in 'Performance & Fuel' started by Pikey, Oct 3, 2012.

  1. Pikey

    Pikey Moderator

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    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: Oct 3, 2012
    #1
  2. My4xburbanrocks

    My4xburbanrocks New Member

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    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?
    #2
  3. SurrealOne

    SurrealOne Former Member

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    I had this same question.
    #3
  4. Pikey

    Pikey Moderator

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    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.
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  5. 1953bowtieguy

    1953bowtieguy New Member

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    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.
    #5
  6. SurrealOne

    SurrealOne Former Member

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    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...
    #6
  7. sierra11

    sierra11 New Member

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    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.
    #7
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  8. SurrealOne

    SurrealOne Former Member

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    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!
    #8
  9. sierra11

    sierra11 New Member

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    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: Oct 3, 2012
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  10. My4xburbanrocks

    My4xburbanrocks New Member

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    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.
    #10
  11. Pikey

    Pikey Moderator

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    I put some Sea Foam in last night. Now averaging 16.7 MPG in the city. I have a long drive to class tonight so I will see how it does on the freeway
    #11
  12. phoebeisis

    phoebeisis New Member

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    4mpg- HUGE difference just from AC use.
    Any chance the prevailing winds were different?
    Of course a this should balance out in a roundtrip
    But maybe not-perhaps the wind switched directions on the return trip.
    I would check prevailing winds-you did the 1st trip in a different season -right??
    4mpg- too much for just AC use.

    Another possibility- are these mpg numbers 22 vs 17.9 the NAV readings or actual distance over gallons pumped in?
    I wouldn't expect the NAV to change over time like that
    BUT if you did actual measurements- distance over gallons pumped in
    Small differences in tank filling-say 1/2 to 1 gallon more filled-can make a HUGE difference in calculated mpg- especially on short trips 150 miles or so??

    Well just some ideas- 4 mpg- the truck still runs well and you are still getting good city mpg
    Makes me think nothing wrong with truck-and you don't have any codes
    Maybe winds maybe just a filling difference maybe AC use maybe just an unlucky combination of winds AC filling difference(if you weren't using onboard mpg gauge)

    No codes- still running well -same city mpg- means nothing wrong with truck
    Wind AC filling difference "bad gas" lower octane than claimed so less advance possible hurts mpg

    Pure guesses of course-post your next trip-curious about anything to do with mpg-I'm a mpg nut- mild hypermiler-98 Suburban(very good -21mpg- pure hy mpg-city 12-13 mpg wife kills my city mpg) 2006 Prius
    Charlie
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  13. Pikey

    Pikey Moderator

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    ok, time resurrect an old thread. My highway mpg is staying at 17-18. I was getting 19-21mpg. Here is the odd thing, my city mpg is at 16-17mpg with a light foot. So, my city has stayed the same but, my highway has decreased. No new mods. Any new ideas on this? thanks guys.
    #13
  14. SurrealOne

    SurrealOne Former Member

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    Tire pressure change due to colder weather? Winter fuel mix finally arrive at a fuel station near you? Time for a tune up? Observer error?
    #14
  15. phoebeisis

    phoebeisis New Member

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    I'm with Surreal
    fuel changes- tire pressure- just some fill to fill variation
    BUT in the winter MPG goes down-but usually city mpg goes also
    #15
  16. TimTom64b

    TimTom64b New Member

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    Try adding some octane booster to it and see if gets better. Also, with the colder weather the engine will utilize more fuel until it reaches running temp.
    #16

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