Solved my bad MPG

Discussion in 'Gen 2 Prius Fuel Economy' started by JC91006, Nov 13, 2017 at 9:12 PM.

  1. JC91006

    JC91006 Senior Member

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    I've been having some bad gas mileage on my 2005 Prius and for the longest time, I haven't been able to figure the problem. One of my 2005 cars would get 52mpg while the problematic one only gets 40mpg on a good day. Both are maintained meticulously and everything is up to date. Only difference, the 40mpg car has 17" wheels. But that should not reduce the mpg by 12 miles.

    I went and checked the spark plugs on the problem car and realized they had new Denso plugs in them, ones I bought from the dealership when I got the car a year ago. Just for fun, I decided to put in some old NGK spark plugs to see if things would change. Drove the car a couple days now and I'm up to 47mpg with the OLD PLUGS. Same driving pattern, nothing has changed.

    I'm not sure why the Toyota Denso plugs would do this, I have no way to explain it. But now at least my cars are back to normal.
     
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  2. dolj

    dolj Senior Member

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    That is interesting.

    What was the gap, comparing old to new?

    Possibly a dud set of Denso plugs, or may be just one dud?
     
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  3. JC91006

    JC91006 Senior Member

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    I'm not sure of the gap, I haven't seen my gapping tool for years. They all seem to be pretty consistent visually.

    I haven't noticed any misfires on techstream but every once in a while, I would have a slight hesitation when I accelerate first thing in the morning. Like it'll accelerate for 1 second and fall off and continue accelerating. Then all is normal after that.

    The new plugs I took out are attached. I'll measure them and see if they are out of spec 20171113_194551.jpg
     
    #3 JC91006, Nov 13, 2017 at 10:52 PM
    Last edited: Nov 13, 2017 at 11:21 PM
  4. fuzzy1

    fuzzy1 Senior Member

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    I hope it stays that way, instead of reverting back to the bad MPG.

    I've witnessed numerous fixed problems turn out to not really be fixed.
     
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  5. JC91006

    JC91006 Senior Member

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    Yeah I hope so. I have no explanation for it.
     
  6. 05PreeUs

    05PreeUs Active Member

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    Heat range would be my guess. You cannot hear the engine knock (pre ignition) until it is really severe, but even a very mild knock robs serious power because it is trying to push one (or more) DOWN when it still wants to move UP.

    I learned decades ago on High Performance 2-strokes which are VERY heat range sensitive, that "does fit", "will fit" and "equivalent" are not the same as being RIGHT for the application. Even minor changes in heat range (same application but different brand) can lead to serious and immediate engine failures.
     
  7. Mendel Leisk

    Mendel Leisk Lapsed Cargo Cultist

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    They look to be the spec'd denso SK16R11:

    upload_2017-11-14_16-52-16.png
    upload_2017-11-14_16-53-24.png
    Yeah maybe the gap?

    FWIW, our previous Civic Hybrid had 2 plugs per cylinder, and when I pulled them, there was one NGK and one Denso per cylinder.

    Strange.
     
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  8. 05PreeUs

    05PreeUs Active Member

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    0.043" is spec, replace at 0.047" or before, but no manufacturer recommends adjusting the gap on double platinum or iridium plugs as the electrodes are VERY easy to damage.
     
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  9. JC91006

    JC91006 Senior Member

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    Not much room for error
     
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  10. 05PreeUs

    05PreeUs Active Member

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    Considering it can take 100k plus to change 0.003", plenty ;)
     
  11. Mendel Leisk

    Mendel Leisk Lapsed Cargo Cultist

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    Those centre posts hardly wear. If it's off, probably because someone knew better. :rolleyes:
     
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