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Sudden Significant Drop in MPG!!! 50 to 40 overnight!

Discussion in 'Gen 4 Prius Fuel Economy' started by PappyAdkins, Jun 21, 2022.

  1. PappyAdkins

    PappyAdkins Junior Member

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    OK fellow owners... First off, I'd like to say, This is my second personally owned Prius... My dad also had/has 2 Prius vehicles!!! I'm saying all this because most the answers I've read seems to 'insult the owners intelligence, in my book)... I'm not asking a stupid question, my car has 80k miles & I have personally monitored all of these miles! my commute for the past 6 years have been the same 7.5 miles everyday through all 24 seasons (well Texas really doesn't have 4 seasons, but you get my point!) I've always used low grade ("up to 10% Ethanol) fuel, usually from Murphy's or Brookshire's. I have always averaged 50+ MPG.... UNTILL my last fill-up!!!! I had heard that pumps have been checked in our area with over 20% ethanol (maybe yes, maybe no) so I thought I'd fill on up (6 gallons) of Ethanol FREE 93 octane & hopefully see my numbers return... there hasn't really been any increase from the now 39-41 MPG each day!!!! So I've even really ruled out the Ethanol issue, if it was even a valid concern.... My 12v battery is a few months new, my tires are also a few months new.... I've also noticed, my 'pulsing' the pedal doesn't trigger the EV mode like it use to... When I come to a complete stop it'll switch to EV mode... but doesn't seem to stay in EV mode like it always has.... Here's my question, does anyone have experience with same issue or have any points of interest to help fix issue.... Thanks in advance...
     
  2. The Professor

    The Professor Senior Member

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    There are a lot of things that can keep the car out of EV mode. Some of these are... cabin temperature colder than the temperature set on the climate control, low (2 or 3 bars) EV battery, high electrical drain (e.g. heated seats), engine not up to temperature, heaving braking demand, sudden braking, repeated braking in quick succession, being stopped on an incline, hybrid battery too hot (check the EV battery air filter in the cabin is clean and not blocked) or too cold, too much accelerator demand... I've probably forgotten tons of them.

    Anyway... if it genuinely feels different, I'd definitely check the cabin EV battery air filter, given the weather.
     
  3. Mendel Leisk

    Mendel Leisk Book Cover Judge

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    Them new tires?
     
  4. Doug McC

    Doug McC Senior Member

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    You are probably already aware of this but I’ll mention it anyway, that wonderful Texas heat and humidity could be playing a part in this, especially if the hybrid battery filter needs cleaning or something is blocking the filter intake grill (restricting air flow into the battery). There is also a air duct underneath the front passenger seat that needs to be clear for the cool air to have a good shot at the filter inlet.
    I mention all of this since I just noticed that with the higher temperatures here even in Illinois that my 2022 has a tendency to favor the ICE over the EV mode more than in cooler temperatures, even without the a/c running.
    The other thing I noticed is you didn’t mention cleaning the throttle body, which with the non-TopTier gasoline you’ve been running is more likely to need it. FWIW
     
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  5. Doug McC

    Doug McC Senior Member

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    Have you checked your brakes? Could they be dragging slightly (an easy check would be to check the wheel temperatures). I mention this because a few years ago when I lived in your area with a different vehicle, my brake fluid was bad (I obviously didn’t know it at the time) and in the summer heat the brakes started dragging.
     
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  6. Mendel Leisk

    Mendel Leisk Book Cover Judge

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    Yeah raise the rear and see how free the wheels spin (with parking brake off, of course...). There's a link in my signature, how freely they should spin, and why they might not. It's says 3rd gen, but equally applicable to 4th I believe.
     
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  7. TheLastMojojomo

    TheLastMojojomo Active Member

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    It sounds like you may be describing my Low MPG Mechanism that I call WEAK and MAX EV States. it's kind of a schizo-post but has lots of useful information that I am in the process of refining.

    Basically what happens is after a 12v disconnect or ECU reset that erases certain learned parameters... the Prius significantly reduces EV mode output to about half of what it should... supplementing Combustion Engine Torque in place of what normally would be supplied by the Electric Motors in EV mode. Until such parameters are re-learned by the ECU's... the Prius will stay this way permanently

    I still don't know exactly why this happens or how to get the parameters to relearn but I am in the process of making a video about this.

    What's strange about your post though is that it doesn't sound like the 12v was disconnected beforehand so maybe it's something different.

    If you are in a very hot climate... a similar EV mode limitation will be placed if it the HV Battery is overheating so that could be the cause as well.

    Posted via the PriusChat mobile app.
     
  8. bisco

    bisco cookie crumbler

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    papppy?
     
  9. PappyAdkins

    PappyAdkins Junior Member

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    first off thanks for reading my post. I did change the 12 volt battery a few months before. So I’m not sure if it’s the same; but please keep me updated. It seems to me it’s a computer problem. The EV light doesn’t work the same as before. It’s hard to explain, but you just KNOW when something doesn’t feel right.
     
  10. TheLastMojojomo

    TheLastMojojomo Active Member

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    Has your low mpg been occuring since roughly the same time as the 12v Disconnect? This would give good evidence to that it is related to the 12v disconnect.

    You can also see this Imgur post with screenshots of others effected by this Low MPG Mechanism.

    Posted via the PriusChat mobile app.
     
  11. PappyAdkins

    PappyAdkins Junior Member

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    No sir, sorry on my gen 4, at least, I changed the 12 volt probably 6 months ago. With that being said, my battery DIED (no juice- none!!!). I even thought something was wrong with the charging system from the way it was acting! But once I replaced the 12 volt, all was great until a couple of weeks ago. We even camped out in the car since then, no issues - no mpg loss!!!
     
  12. bisco

    bisco cookie crumbler

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    are the brakes okay?
     
  13. PappyAdkins

    PappyAdkins Junior Member

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    Brakes good. It was a sudden change, pretty sure it’s computer caused. It’s not using EV mode like it normally had!!!! I think TheLastMojojomo has pinpointed the issue, now Toyota needs to come up with a solution to correct the issue.
     
  14. bisco

    bisco cookie crumbler

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    no doubt they are working hard at it. all the best!(y)
     
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  15. TheLastMojojomo

    TheLastMojojomo Active Member

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    Are you sure it's not the Texas heat? I also stated in my original reply that it could be the heat.

    If you are experiencing temps in the 100's or close to it with direct sunlight all day... the Hybrid Battery can easily overheat.

    When the Hybrid Battery overheats EV mode will be greatly throttled back or stopped completely along with regenerative braking.

    On my Gen 2 NiMh HV Battery... this starts at 130°F.

    Have you used Dr. PRIUS or a similar OBDII app like Torque to monitor HV Battery temps? Just something to rule out.

    Posted via the PriusChat mobile app.
     
    #15 TheLastMojojomo, Jul 1, 2022
    Last edited: Jul 1, 2022
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  16. PappyAdkins

    PappyAdkins Junior Member

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    Doubt that, always been in Texas, Daily Commute for many years, we’ve even had ‘cooler’ days since this begun. My thinking is my car has survived a lot of Texas summer days before without any issues! Besides that my wife’s and my dad’s Prius are driving right next to mine without issues.
     
  17. bisco

    bisco cookie crumbler

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    i wonder if some of the cells have become weak, might be worth a health check
     
  18. Kramah313

    Kramah313 Active Member

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    I’ve got a gen3 but run scangauges to monitor the battery temps. There is a gauge called BT2 that checks one of 3 battery temp sensors - this one seems to usually be the hottest and triggers the car to do different things. Above 104, the fan kicks on. Above somewhere around 120, the car goes into a protect mode where it won’t charge or use more than about 20 amps at a time. I don’t know what these values are for the gen 4, but I’d be willing to bet that has something to do with it as the last couple weeks I’ve seen as high as 124 even pulling out of the garage and often start my drive in protect mode. I’m also down about 5 mpg at least.

    Only thing I can tell you to try is parking in the shade or using sun shades in the car. If you’re already doing this, it’s probably just an unfortunate part of the Texas heat. One thing I have noticed is that the car’s internal temperature in terms of how hot it feels to me doesn’t always correlate with this value, instead it’s how much sunlight the battery compartment in the trunk is getting that seems to matter most.
     
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  19. Doug McC

    Doug McC Senior Member

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    Have you checked the dates and vehicles that were involved in the posts TheLastMojojomo sent you to?
    Have you even taken it in to have your theory checked? Perhaps you are right, but then perhaps not. You wanted suggestions as to the cause but don’t seem to have followed up on any of them (such as checking your brakes, which in the Texas heat combined with bad brake fluid are legitimate possibilities, (“brakes good”???? What does that mean? Have you checked them or had them checked since this started?)).

    Don’t mean to come across as overly critical but you are accusing Toyota of ignoring a problem that, at this point, you only suspect might exist, (and that suspicion SEEMS to be based only on something you read on the internet) and haven’t even turned to them in checking it out.
    Please correct my observations if I am missing something here.
     
    #19 Doug McC, Jul 1, 2022
    Last edited: Jul 1, 2022
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  20. Doug McC

    Doug McC Senior Member

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    OK: I’m confused: When did you camp out in the car? After you changed out the battery, right? Was it before the sudden drop in mpg or after the drop?