Lower mileage after spark plug change?

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by geeman1082, Aug 7, 2014.

  1. geeman1082

    geeman1082 Junior Member

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    Hey, has anyone else experienced lower mileage after changing the plugs at about 120k miles?

    In the 10 months (23.6k miles) since I had the dealer change the plugs, I have averaged 43.3 mpg overall.

    In the same 10 months a year earlier (to remove as much seasonal effect as I could) (21.9k miles), I averaged 45.2 mpg.

    I commute about 85 miles / day, so the vast majority of my miles are on the same 42-mile stretch of road.

    I was sure that I was getting reduced mileage, but I wanted to wait a length of time to make it more statistically significant.

    I understand that it's possible that other factors are reducing the mileage, but I'm following the maintenance recommendations so it's still somewhat disappointing.
     
    #1 geeman1082, Aug 7, 2014
  2. JC91006

    JC91006 Senior Member

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    It probably has to do with your tires wearing down a bit, causing the mileage difference. If you get new tires, it should go back up
     
    #2 JC91006, Aug 7, 2014
  3. uart

    uart Senior Member

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    ??? Normally the effect of wearing of tires would be the exact opposite.
     
    #3 uart, Aug 8, 2014
  4. wjtracy

    wjtracy Senior Member

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    I just replaced my plugs DIY last month at 135k miles.
    Not seeing an obvious difference.

    I assume the dealer used the spec iridium plugs.
    On a Gen2 its easy to replace plugs so worse case you could buy on Amazon $8/ea and re-replace.

    But we don't hear too much chat here about plugs causing low MPG.
    Your MPG seems a little low for that long commute...is it high speed?
    Could be tires what climate are you in? What tires?
     
    #4 wjtracy, Aug 8, 2014
  5. taylor_priusII

    taylor_priusII New Member

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    Lots of factors including A/C use. That decimates MPG...in the same way that Obama has decimated the Constitution...
     
    #5 taylor_priusII, Aug 8, 2014
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  6. dolj

    dolj Senior Member

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    "Decimates" is an extreme description. Except in extremely hot temperatures, I haven't found using the A/C to be much of a hit, 1 or 2 mpg at the most. Much more impact by under-inflated tires or driving technique. Just my observation.
     
    #6 dolj, Aug 8, 2014
  7. bisco

    bisco cookie crumbler

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    agreed, i still see bits of the constitution around.:cool:
     
    #7 bisco, Aug 8, 2014
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  8. Jon Hagen

    Jon Hagen Active Member

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    A common (slight) problem with any spark ignition engine. Has to do with the sharp edges on the electrodes on the new plugs firing at a lower voltage and less intense spark than slightly worn electrodes that have all the sharp edges worn round.
    The rounded smooth electrodes are a harder path for the spark to jump, so the coil has to build a higher voltage, more intense spark to jump the gap. If the correct plugs ,set to the correct gap were installed, the problem should fix itself in a few thousand miles.
    like new tires, it will work better when they break in.:)
     
    #8 Jon Hagen, Aug 8, 2014
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  9. bisco

    bisco cookie crumbler

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    my guess is either incorrect plugs or gap. pull one out for the fun of it. maybe they never changed them and gave you a placebo.
     
    #9 bisco, Aug 8, 2014
  10. Former Member 68813

    Former Member 68813 Senior Member

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    My guess is the very cold winter in the last 10 months pulled the average down.
    It would be unusual for different plugs to affect MPG in a measurable way (outside a lab), unless wrong/defective plugs were installed. Did they provide a part number in the bill?
     
    #10 Former Member 68813, Aug 9, 2014
  11. mohamed dhalla

    mohamed dhalla New Member

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    Hi i have 07 prius with high milage
     
    #11 mohamed dhalla, Aug 10, 2014
  12. bisco

    bisco cookie crumbler

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    when you say 'high mileage', do you mean mpg's or total miles accumulated?
     
    #12 bisco, Aug 10, 2014
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