sudden big drop in MPG

Discussion in 'GS Boxers' started by trc.rhubarb, Nov 18, 2012.

  1. trc.rhubarb

    trc.rhubarb ZoomSplat!

    Joined:
    Nov 15, 2011
    Oddometer:
    2,998
    Location:
    Concord, CA
    Bike has ~52.5k miles on it and runs great. It's a bit noisy and has been since I picked it up a year ago. One day I'll get around to ordering new cam chain tensioners.

    Anyhow, I've generally gotten in the high 30's and sometimes even mid 40's for mileage but the last 3 tanks of gas (over the last 5 days) have been in the low low 30's. Even the bike knows something is up and shows 31.x mpg on the screen after a reset of the average.

    It runs well and accelerates fine, no carbon build up on the bags or muffler.

    air filter changed, plugs checked, valves & tb were done and checked good at 48k. FD fluid was changed then and there is no freeplay. No noises in the driveline when rolling in neutral. Checked for drag on front and rear wheels. Brakes aren't hot after riding. New rear tire was put on just before the issues started but the tire looks good and is still at 40psi; front good too. The only other change is that it's been raining.

    What's next to check?
    #1
  2. LaurelPerryOnLand

    LaurelPerryOnLand Long timer

    Joined:
    Jun 9, 2009
    Oddometer:
    3,482
    Location:
    Denver, CO
  3. Rinty

    Rinty Been here awhile

    Joined:
    Jun 3, 2011
    Oddometer:
    590
    Location:
    Calgary, Alberta
    Oxygen sensors.

    But I would just have the whole system scanned.
    #3
  4. trc.rhubarb

    trc.rhubarb ZoomSplat!

    Joined:
    Nov 15, 2011
    Oddometer:
    2,998
    Location:
    Concord, CA
    Thanks, I think too much for it to just be the cold. In fact, since it's been raining, it's not been that cold.
    O2 sensors will be interesting to test. I guess I'll see if it gets better first.
    #4
  5. kk3an

    kk3an Adventurer

    Joined:
    Jul 27, 2009
    Oddometer:
    84
    Location:
    East Berlin, PA
    I had the same symptoms at about 38,000 miles, and replacing both oxygen sensors made the mileage pop back up.

    Looking back, I'm not sure if just a simple cleaning of the sensor area itself will cure this or not, or if a total replacement is needed.

    Dan
    #5
  6. tjt94

    tjt94 Been here awhile

    Joined:
    Jun 21, 2008
    Oddometer:
    610
    Location:
    Hell Centro, CA (8 miles to Mexico)
    It is because of winter blend fuel. I always lose about the same is California is the winter.
    #6
  7. El Gato

    El Gato Been here awhile

    Joined:
    Jun 7, 2010
    Oddometer:
    813
    Location:
    Los Gatos, CA
    That was my initial thought as well, but CA switched to winter gas early this year -- about 6 weeks ago on an exemption from the governor due to the Richmond refinery fire.
    #7
  8. tjt94

    tjt94 Been here awhile

    Joined:
    Jun 21, 2008
    Oddometer:
    610
    Location:
    Hell Centro, CA (8 miles to Mexico)
    That may have been how long it took to deplete the tanks. Regardless, the difference in actual cost between they mileage figures is not that much so keep an eye on it. If it is running fine just ride. Good luck.
    #8
  9. ToddLite

    ToddLite Been here awhile

    Joined:
    Oct 15, 2012
    Oddometer:
    124
    Location:
    San Diego, CA
    Interesting - I just reset the MPG calculator yesterday and noticed I was pulling at best 37-39 MPG doing some simple highway cruising. Definitely recall getting much better than that here in SoCal a few months ago. F'n winter gas indeed..
    #9
  10. trc.rhubarb

    trc.rhubarb ZoomSplat!

    Joined:
    Nov 15, 2011
    Oddometer:
    2,998
    Location:
    Concord, CA
    Bike is running well just bad mileage. I don't ride my other so much but I think I'll switch for a week and see if it's getting bad mileage too. That would show for sure it's the gas. Thanks for the input!
    #10
  11. Tmaximusv

    Tmaximusv Long timer

    Joined:
    Jun 19, 2012
    Oddometer:
    1,617
    Location:
    Gateway to the Sandhills
    I scanned this thread earlier and didn't have anything to offer...I just remembered something to offer.:D

    If your gas cap isn't sealing properly, you can lose about 40% of your fuel to evaporation (under warm conditions) DAMHIK.
    #11
  12. trc.rhubarb

    trc.rhubarb ZoomSplat!

    Joined:
    Nov 15, 2011
    Oddometer:
    2,998
    Location:
    Concord, CA
    O'ya, went through that one befure :) Funny how you can keep smelling gas and 'wonder where it's coming from'
    #12
  13. ukdave

    ukdave Been here awhile

    Joined:
    Dec 12, 2007
    Oddometer:
    161
    I had a leaks from fuel pipe connections more than once.
    #13
  14. trc.rhubarb

    trc.rhubarb ZoomSplat!

    Joined:
    Nov 15, 2011
    Oddometer:
    2,998
    Location:
    Concord, CA
    Good one to check. Even though I'm not smelling gas, it could be leaking. Hopefully my new shocks arrive this week and I can just take the whole thing apart.
    #14
  15. Multiplicity

    Multiplicity Been here awhile

    Joined:
    Nov 12, 2012
    Oddometer:
    172
    Location:
    Nor-Cal
    You should also treat the fuel with Techron every 10K miles to keep the injectors clean.

    Buy a bottle at the local parts store :pynd
    #15