Solved my bad MPG

Discussion in 'Gen 2 Prius Fuel Economy' started by JC91006, Nov 13, 2017.

  1. JC91006

    JC91006 Senior Member

    Joined:
    Nov 10, 2013
    11,663
    4,113
    0
    Location:
    Los Angeles, CA
    Vehicle:
    2005 Prius
    Model:
    II
    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.
     
    Data Daedalus and Aaron Vitolins like this.
  2. dolj

    dolj Senior Member

    Joined:
    May 14, 2012
    2,615
    1,130
    0
    Location:
    Wellington, New Zealand
    Vehicle:
    2006 Prius
    That is interesting.

    What was the gap, comparing old to new?

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

    JC91006 Senior Member

    Joined:
    Nov 10, 2013
    11,663
    4,113
    0
    Location:
    Los Angeles, CA
    Vehicle:
    2005 Prius
    Model:
    II
    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
    Last edited: Nov 13, 2017
  4. fuzzy1

    fuzzy1 Senior Member

    Joined:
    Feb 26, 2009
    10,570
    4,316
    90
    Location:
    Western Washington
    Vehicle:
    2012 Prius
    Model:
    Three
    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.
     
    uart, Data Daedalus and scona like this.
  5. JC91006

    JC91006 Senior Member

    Joined:
    Nov 10, 2013
    11,663
    4,113
    0
    Location:
    Los Angeles, CA
    Vehicle:
    2005 Prius
    Model:
    II
    Yeah I hope so. I have no explanation for it.
     
  6. 05PreeUs

    05PreeUs Active Member

    Joined:
    Oct 1, 2016
    792
    351
    0
    Location:
    Canton, TX
    Vehicle:
    2005 Prius
    Model:
    N/A
    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.
     
    AzWxGuy likes this.
  7. Mendel Leisk

    Mendel Leisk Lapsed Cargo Cultist

    Joined:
    Oct 17, 2010
    25,365
    14,674
    80
    Location:
    Greater Vancouver, British Columbia, Canada
    Vehicle:
    2010 Prius
    Model:
    Touring
    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.
     
    Data Daedalus and JC91006 like this.
  8. 05PreeUs

    05PreeUs Active Member

    Joined:
    Oct 1, 2016
    792
    351
    0
    Location:
    Canton, TX
    Vehicle:
    2005 Prius
    Model:
    N/A
    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.
     
    JC91006 likes this.
  9. JC91006

    JC91006 Senior Member

    Joined:
    Nov 10, 2013
    11,663
    4,113
    0
    Location:
    Los Angeles, CA
    Vehicle:
    2005 Prius
    Model:
    II
    Not much room for error
     
    SFO likes this.
  10. 05PreeUs

    05PreeUs Active Member

    Joined:
    Oct 1, 2016
    792
    351
    0
    Location:
    Canton, TX
    Vehicle:
    2005 Prius
    Model:
    N/A
    Considering it can take 100k plus to change 0.003", plenty ;)
     
  11. Mendel Leisk

    Mendel Leisk Lapsed Cargo Cultist

    Joined:
    Oct 17, 2010
    25,365
    14,674
    80
    Location:
    Greater Vancouver, British Columbia, Canada
    Vehicle:
    2010 Prius
    Model:
    Touring
    Those centre posts hardly wear. If it's off, probably because someone knew better. :rolleyes:
     
  12. audiodave

    audiodave Member

    Joined:
    Feb 3, 2017
    174
    62
    0
    Location:
    US
    Vehicle:
    2004 Prius
    Model:
    Five
    Curious if you swapped the Denso back and milage dropped or you left the NGK in?
     
  13. JC91006

    JC91006 Senior Member

    Joined:
    Nov 10, 2013
    11,663
    4,113
    0
    Location:
    Los Angeles, CA
    Vehicle:
    2005 Prius
    Model:
    II
    I haven't touched it since. With winter and heat use, the mpg now is around 42mpg. Barely over 40
     
  14. audiodave

    audiodave Member

    Joined:
    Feb 3, 2017
    174
    62
    0
    Location:
    US
    Vehicle:
    2004 Prius
    Model:
    Five
  15. audiodave

    audiodave Member

    Joined:
    Feb 3, 2017
    174
    62
    0
    Location:
    US
    Vehicle:
    2004 Prius
    Model:
    Five
    Did you disconnect the battery and reset the computer when you did the plug swap?

    Posted via the PriusChat mobile app.
     
    uart and JC91006 like this.
  16. JC91006

    JC91006 Senior Member

    Joined:
    Nov 10, 2013
    11,663
    4,113
    0
    Location:
    Los Angeles, CA
    Vehicle:
    2005 Prius
    Model:
    II
    Yes I do this every time I change plugs and clean throttle body/MAF sensor
     
  17. Mendel Leisk

    Mendel Leisk Lapsed Cargo Cultist

    Joined:
    Oct 17, 2010
    25,365
    14,674
    80
    Location:
    Greater Vancouver, British Columbia, Canada
    Vehicle:
    2010 Prius
    Model:
    Touring
    Interesting. You're thinking it forces a recalibration of sorts?
     
  18. uart

    uart Senior Member

    Joined:
    Sep 7, 2009
    4,206
    1,140
    0
    Location:
    Australia
    Vehicle:
    2005 Prius
    I'd like to add here that when I replaced my spark plugs at 200,000 km, Sparkplug replacement at 200,000 km (pictures). | PriusChat the OEM ones that I removed were the NGK IFR5T11 and the ones I replaced them with were Denso SK16R11.

    There was no loss of performance or MPG, and at 320,000 km the Densos are still performing as they should.
     
  19. audiodave

    audiodave Member

    Joined:
    Feb 3, 2017
    174
    62
    0
    Location:
    US
    Vehicle:
    2004 Prius
    Model:
    Five
    It definitely changes things for a tank or two of gas before it's calibrated.

    Posted via the PriusChat mobile app.
     
  20. uart

    uart Senior Member

    Joined:
    Sep 7, 2009
    4,206
    1,140
    0
    Location:
    Australia
    Vehicle:
    2005 Prius
    Yes, it's still a bit of a mystery (well at least to me) of exactly how it does it, but the ecu that tracks fuel consumption somehow uses "historical data" to tweak the accuracy of the raw fuel consumption data taken from the fuel injector pulses.

    The raw fuel consumption data essentially just counts up the pulse widths of all of the fuel injector pulses, and assumes that they all deliver some nominal amount of flow. In practice though, the open loop behavior of the fuel injector might be a little bit leaner or a little bit richer than the nominal value and this throws off the calculation. If the actual injector flow is a little more than it's nominal value, then more fuel is delivered than what the ECU counts, so you get an overly optimistic calculation (this one seems to be the most common). If the fuel injectors deliver a little less than the nominal amount then the ecu counts more fuel than is actually used and calculates an overly pessimistic consumption (my Prius actually does this one).

    While I'm not exactly sure of what data or what feedbacks it uses, over time the ecu learns and adjusts for the fuel injector inaccuracies and gives a more refined calculation. It can take quite some time (several full tanks worth at least), but most people report that it usually settles to a calculation that is just a little bit optimistic, but still reasonably accurate.
     
    #20 uart, Jan 26, 2018
    Last edited: Jan 26, 2018
    Mendel Leisk and SFO like this.
Loading...