Any thoughts on what to check for low MPG?

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by '04Prius, Jan 8, 2021.

  1. '04Prius

    '04Prius Junior Member

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    I’ve got a 2004 Prius with 195,800 miles on it, and I’m taking it on a cross country highway trip (cruise control at 75mph, no hills) and my MPG is in the 34-36 range. This seems low considering it used to get 42-47 in the same conditions, and I’m at a little bit of a loss as far as what I should check. Here’s what’s been done so far:

    HV battery replaced with one from NewPriusBatteries.com 400 miles ago

    12v battery replaced about 800 miles (about a month ago). Voltage at 11.9V with car in accessory mode with headlights on.

    Struts and control arm bushings at 190,000, along with an alignment done at that time

    Tires were replaced at 190,000 with Michelin Defenders, running at 42PSI all the way around. (I knew I’d take an MPG hit without low rolling resistance tires, but 10+mpg seems excessive)

    Oil change at 194,800. Level is exactly at the top mark on the dipstick still

    Spark Plugs were done at around 175k, replaced with Bosch Iridium plugs.


    The only thing that’s odd is that batt temp sensor 1 seems to be displaying a value about 10-15 degrees less than the other sensors, so I’m wondering if I didn’t clip it properly when I replaced the battery pack. However no warning lights are on. Does anyone happen to know which sensor sensor 1 is, and would this cause a dramatic decrease in MPG? I’m probably going to end up pulling the battery pack again to check the sensors.

    I’d appreciate any ideas.

    Thanks!
     
  2. JC91006

    JC91006 Senior Member

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    You have to consider the engine and transmission has 195k miles on it, the wear on the parts will make the car less efficient. I had a 2006 models a few years ago that the previous owner didn't maintain well, the oil was not changed according to the schedule. That car was always registering around 40mpg while my other cars were consistently above 45mpg

    If I were you, I would proabably try changing out the spark plugs and use Denso or NGK plugs. Those are the plug brands that Toyota uses, not the Bosch branded stuff. After you switch those out, then reset all your computers and get a good average AFTER 1000 miles of driving.
     
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  3. jerrymildred

    jerrymildred Senior Member

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    Agree that just the miles on the machine can be a real factor. For one thing, compression probably isn't what it used to be. And the factory spec plugs might help, too. Perhaps the VVT is getting sluggish. You might want to clean the MAF just for the fun of it. A dirty one will throw off the fuel mixture.
     
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  4. Skibob

    Skibob Senior Member

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  5. '04Prius

    '04Prius Junior Member

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    So it finally threw a P0A9B code. I think that might be what’s causing it. I was able to get 45+mpg in February before the battery issues started, so I don’t think that the wear on the engine and transmission has increased all that crazy much in the 5k miles I’ve put on it since then. I might look into the fuel/air sensors after I fix the battery sensor. I’ll take care of that and then report back.
     
  6. Skibob

    Skibob Senior Member

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  7. edthefox5

    edthefox5 Senior Member

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    You may not have taken the ecu box apart and inspected the board for corrosion or whiskers on the pins nor checked the ecu plugs and sockets.

    how bad was the buss bar corrosion if it was bad you may have corrosion on those plugs.

    it’s a must to interrogate that ecu when you do a NPB install. I was lucky when I did my NPB the ecu was super clean.
     
  8. '04Prius

    '04Prius Junior Member

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    I didn’t take the actual ECU apart, but I did look at the plug and socket, and didn’t see any corrosion. The bus bars are all new, so no corrosion on them. Would corrosion on the ECU cause an erroneous reading on just one sensor? Because it’s only that one that’s reading low. I’m kinda thinking it didn’t get clipped properly and fell to the bottom of the case (I didn’t move them to the top unfortunately) while the battery was getting installed. If I can’t see a sensor lying on the bottom of the case, I’ll pull apart the computer and look at it.
     
  9. '04Prius

    '04Prius Junior Member

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    If it does end up happening to be the ECU, where do you suggest I source a new one? If I just buy one used off eBay, what’s the likelihood hood that it will just have the same issue as my current one?


    Also, I realized I misread your earlier question a bit. Corrosion on the factory bus bars didn’t look excessively bad to me, although there was some discoloration.
     
  10. Prodigyplace

    Prodigyplace Senior Member

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    You made me look up the definition. I did not realize there are two but neither fit your use

    The first definition would involve asking questions of the ECU. The second involves sending it signals and checking for an appropriate response. Neither seem to fit your usage.

    Interrogate | Definition of Interrogate by Merriam-Webster
     
  11. ChapmanF

    ChapmanF Senior Member

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    A quick google for "interrogating the" (including the quotes, to search as a phrase) will turn up a lot of examples where the thing after 'the' is something that you want to learn something from or about, even if it's not a thing you can literally ask questions of or a thing that can answer you. There's a touch of metaphor involved but it is a thing people say.
     
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  12. edthefox5

    edthefox5 Senior Member

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    in the future I won’t use such big words.
     
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  13. '04Prius

    '04Prius Junior Member

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    I understood what you meant regardless.

    Moving off the topic of grammar, the more I drive it the more it wouldn’t surprise me if the ECU is bad. It seems to be not managing the SOC of the battery in a normal manner. (For example it will keep charging the battery at a low current even if it is at the top of the blue bars and I accelerate hard.) I don’t think this would be caused by the temp sensor, even if it isn’t clipped properly (although I will check it).

    if I find corrosion, I need to replace the ECU, correct? And what’s the likelihood that if I get a used one it has failed in the exact same way as mine? Is there a way to use circuit board cleaner or something to “fix” my existing computer?
     
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  14. sam spade 2

    sam spade 2 Senior Member

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    Along that same line, the CAT might be plugging up too.

    If a spark plug makes a proper spark, it is no better or worse than the next brand.

    The entire car is less efficient than it was when new.
    AND....if that "new" hybrid battery is a rebuild, that could be part of the problem too.

    I think this "project" is futile.
     
  15. '04Prius

    '04Prius Junior Member

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    The “new” battery is not a rebuild. It is new with cylindrical cells. Look at the post by 2k1Toaster for more info on the packs he sells.

    I am confident that the pack is not the issue. I think at this point it is either a.) The battery ECU, or b.) An improperly installed or damaged temperature sensor. See my earlier post for the code that tripped.

    I understand that the car is not as efficient as it was when it was new. That is a given. However with the new battery pack, I think it’s reasonable to expect similar fuel mileage to early last year. I’ve only put about 7,000 miles on it since December of last year. I have a hard time believing that a car with a new battery pack and 195,000 miles on it will be so much less efficient than a car with a factory battery and 188,000 miles on it that it loses 10+ mpg.
     
  16. JC91006

    JC91006 Senior Member

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    The battery pack you installed is an aftermarket battery pack, results will not be as consistent as an original Toyota designed product. It's like comparing AA size batteries from Duracell and some aftermarket equivalent. Your results will not be the same
     
  17. '04Prius

    '04Prius Junior Member

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    And that’s fine. I’m not expecting it to be a new car. However the all of the new modules were supposedly tested at 6.5ah with a calibrated tester, which is why I installed the kit that I did, but that’s neither here nor there.

    The low mpg was likely the early symptom of the P0A9B code that eventually tripped. I just made this thread before the code tripped, so I couldn’t put it in the thread title. Perhaps I should make a new thread that’s more representative, but I didn’t really want to spam the forum with threads.

    To be honest, I don’t really care all that much about what gas mileage I get. I just noticed a sudden rapid drop, which told me that something was wrong before the code tripped. The code is now helping to narrow down what is wrong. I want the car to be as reliable as a 200k+mile car can be. If it isn’t as efficient as it was from the factory, I’m okay with that.
     
  18. sam spade 2

    sam spade 2 Senior Member

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    Things work fine, right up to the minute that they DON'T.
    Maybe this came on suddenly OR maybe it was building for a LONG time and you just didn't notice.

    Good luck.
     
  19. sam spade 2

    sam spade 2 Senior Member

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    Maybe they will be better.
    I have stopped using Duracell because of short life expectancy, high price and leaking when discharged.
    In this case, I think there is NO good evidence that the completely rebuilt pack he has is not at least as good as OEM.
    BUT.....I wouldn't blindly assume that it is fine either.
     
  20. Prodigyplace

    Prodigyplace Senior Member

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    I knew you could not send the ECU signals and verify proper response because only Toyota knows that information. Is seems like misuse of the term. Grown up people use the proper terms. Hopefully you learn that before you get to my age.
     
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