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Decreased MPG Post Spark Plug and Air Filter Change

Discussion in 'Gen 2 Prius Fuel Economy' started by Nquinto, Jan 2, 2012.

  1. Nquinto

    Nquinto New Member

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    Hello everyone! Anyone had any problems with MPG going down post new spark plugs and air filter? Plugs were e recommended iridium NGK, and the air filter was a performance STP filter. The plugs were supposedly pre-gapped. When I first changed everything the MPGs went from 45-46 to 32-34. Now after a month I'm at about 37. I have had the car through many winters and different tire pressures and have never had a significant drop in MPGs. I am assuming it is solely related to the change. What could I have done wrong? Any help would be greatly appreciated. I drive about 600 miles a week and it's definitely noticeable. Thanks in advance!
     
  2. wjtracy

    wjtracy Senior Member

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    How many miles on the car? First spark plug change?
    Have you checked 12v battery via the batt check on the MFD?
     
  3. Nquinto

    Nquinto New Member

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    Just about 140,000....First plug change and I did it myself, but i was pretty careful to follow all steps carefully. I have not checked the Batt.... How is this done?
     
  4. wjtracy

    wjtracy Senior Member

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  5. Nquinto

    Nquinto New Member

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    That worked well. Thanks for the link. It appears the battery is ok. No obvious problems on the menu screen. Could this MPG drop be related to improper gaps on the plugs? Or am I reading too far in? Thanks again for all your help!
     
  6. seilerts

    seilerts Battery Curmudgeon

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    It would help if you could be more specific. For example, which particular model of NGK plugs did you use? And "It appears the battery is ok" is the sort of thing that really sets us off, because the numbers are important. People think, "it must be okay if it is 12.0V" when the reality is that a 12.0V reading with no load indicates a deeply discharged battery.

    Is that air filter paper or oiled? How about engine oil, did you install 3.5 quarts of 5w30 on the last change?

    Next time you go out to start the car if it has been sitting for a while, see how long it takes to reach idle shutoff.
     
  7. F8L

    F8L Protecting Habitat & AG Lands

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    Please state the actual battery voltages. The car is not going to just tell you that the battery is bad. :) If the voltage was below 12.4v with no accessories on and after the car has sat for 6+ hours then your battery is weak and should be replaced.

    You stated that the plugs were pregapped but did you check them to ensure they were within spec? Improperly gapped plugs could reduce mpg but they would have to be way off. Have you tried pulling codes to see if you have any misfires?
     
  8. Nquinto

    Nquinto New Member

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    Well the battery was only at 12.0 with no load so that is helpful. I did not see that in the above instructions. From what I read, that was ok. As for the filter it is paper. The oil weight is 5w30. What time frame should I be looking for, for idle shutoff? Thanks for the input, I am learning a lot.
     
  9. Nquinto

    Nquinto New Member

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    I checked the numbers against the above post. Was just eying to keep the reply brief. With no load sitting for 3 hrs the car was at 12.0.....when the car is turned on the volts are at 14.2 with nothing else running. I neglected to check the gaps when I installed the plugs (rookie mistake) but as you said I thought since they were new they should not be way off. I have not pulled any codes, nor do I know much about that. I have heard a rumor that auto zone will check codes for free. Thanks for the input,
     
  10. jdenenberg

    jdenenberg EE Professor

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    You left out a critical part of the check sequence. Add a load while NOT in "READY" mode (e.g. accessories, headlights) and see how far the voltage drops. If noticeably below 12v your battery is suspect. If it continues to drop at a noticeable rate your battery is toast.

    JeffD
     
  11. seilerts

    seilerts Battery Curmudgeon

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    There will not be any codes if you do not have any of the check engine or other warning lamps lit. Well, okay, there might be a pending code, but you'd have to have something serious like a misfire to take the MPG hit.

    I think your 12V battery is weak and should be replaced. Did you have the radio on while you were working on the plugs?

    Idle shutoff: This is the hallmark of most hybrids, where, if the car is not moving, the engine will turn itself off to save fuel and reduce emissions. If you start the car, it should turn itself off within a few minutes, given the warm weather that you bastard Californians are currently enjoying.
     
  12. Nquinto

    Nquinto New Member

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    Ok so tested the battery with car on, but not in ready mode. The volts when down to 11.6......I believe this Is too low.... And I feel like the engine has been running more than normal lately....also for seilerts I checked the time until idle and it was just a smudge under 10 sec......Thanks again for all the input everyone. Is is really great!
     
  13. Nquinto

    Nquinto New Member

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    Ha ha! Ok Seilerts that's what it seems like. I misread the idle test and will re-do it. And yes......the weather has been nice. But no snow means no boarding so that is a Down side. Much appreciated.....cheers!
     
  14. Nquinto

    Nquinto New Member

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    Ok....Anyone have a good recc on a battery for the Prius?
     
  15. richard schumacher

    richard schumacher shortbus driver

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    There are three choices:
    - another identical Toyota battery
    - Optima DS46B24R YellowTop
    - Optima D51 with adapter kit from eLearnaid Toyota Prius 12 Volt Auxiliary Battery with install kit for 2004 - 11

    The two Optimas are identical except for post size, which is why eLearnaid sells an adapter kit. There are no other direct drop-in replacements.
     
  16. Nquinto

    Nquinto New Member

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    Thank you Richard! Appreciate the help....Thought I could get away with a cheaper one but I know it comes down to ya get what you pay for!
     
  17. uart

    uart Senior Member

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    Why not the Toyota OEM filter?
     
  18. uart

    uart Senior Member

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    Yes that's pretty low, but I wonder why it would go bad (as something is now different) just after you changed the plugs? Can you think of anything that may have drained the battery, like seilerts suggested about the radio for example. You could try giving the battery a 12 hour slow charge and see if it improves.
     
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  19. Pdub

    Pdub Junior Member

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    Nquinto,
    I want to know what happened. Did the new battery fix your FE problem?
     
  20. 9G-man

    9G-man Senior Member

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    If the 12v battery has not been replaced, it needs to be. That will help alot. Get the Optima with proper post size.
    Remove and clean your MAF sensor. Very easy, buy some MAF sensor cleaner, unplug it, remove 2 small phillips head screws. Pull it out. Spray the temp sensor bulb and "hot wires" of the sensor. Reinstall and plug in.
    While you have it out I'd clean the throttle body throat with TB cleaner too, while its easy to get to.
    Your Prius will run like new and be back to great mileage.
    You did get the spark plugs tight, right
    Don't worry about the gap.