SUDDEN drop of about 10MPG

Discussion in 'Gen 4 Prius Care, Maintenance and Troubleshooting' started by Zardoz, Jul 11, 2019.

  1. Zardoz

    Zardoz Member

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    This is going to sound protracted... so be it. But, PLEASE try to read through it and give me your thoughts.

    About two months ago IMMEDIATELY after my last service (24,000 mile) my car has suffered a significant drop in gas mileage PLUS the engine now acts weird.

    I ALWAYS drive in Eco mode unless I am in the mountains. I am a creature of habit. I drive the same roads at the same speeds.

    My symptoms:
    1.While cruising at a set speed of 75 mph, I came upon a slower vehicle and of course my car slowed down to match it. I actually used the brake as I wanted to slow faster.
    2. Upon pressing resume on the cruise stick and with the highway at a SLIGHT up angle I noticed the car NEVER got to speed. It basically reached this point of acceleration on the display where the speedo was not increasing. I drive this road a LOT. I have never seen this issue before. I took it out of Eco and into Normal and it recovered.
    3. Several days later I am driving 9.2 miles from a hospital in downtown Phoenix towards my home the outside temperature indicator shows 100 degrees. For the entire trip the engine NEVER shut off. On some occasions sitting at traffic lights the engine would race. When I got home the battery showed fully charged. VERY unusual. I use the single drive indicated gas mileage all the time. That trip normally gives me about 58 - 60 mpg. I got 35 mpg!
    4. Driving a segment of highway 101 at 65 mph from Scottsdale to Phoenix I usually get about 65 mpg. I am routinely NOW getting about 53 IF I am lucky!
    5. Driving from downtown Phoenix up the highway 51 to the highway 101 TODAY I got 48. I usually get about 55 mpg.
    6. I think it is ALL an engine / engine electronics problem. The engine seems sluggish. It is as if a sensor is failing or has a loose connection but not bad enough to trigger the diagnostics unless item 3 above was some form of auto calibration. If so, it failed. Perhaps a dirty O2 sensor, dirty mass air flow sensor?

    Any ideas most gratefully appreciated!

    Thanks!
     
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  2. Leadfoot J. McCoalroller

    Leadfoot J. McCoalroller Senior Member

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    Sounds more like the engine is doing fine, and the computer is limiting electrical participation- so it's forcing the gas engine to do more of the work.

    The computer will limit the hybrid electric contribution if the battery temperature is too high. Normally the battery is cooled by a fan that draws air from the passenger cabin. The battery favors the same range of temperatures that most people like, so for the climate you describe it would likely be air conditioned down to 70-odd degrees.
     
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  3. The Professor

    The Professor Active Member

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    Had this happen to me for that very reason.

    You can tell if it's this as the battery fan in the cabin will be audible. Normally it won't be.

    Or, maybe the battery fan has failed/blocked, but I think that triggers a warning.
     
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  4. bisco

    bisco cookie crumbler

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    what services did they perform at 24k?
     
  5. jerrymildred

    jerrymildred Senior Member

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    If it's like the Prime, the A/C can be set to front only. You might need to turn that feature off to get enough cool air back to the battery in that kind of heat. Also, take the A/C out of ECO mode. I agree with @Leadfoot J. McCoalroller that it's most likely a hot battery. If the car sat in the sun for a while, the battery might be hotter than a pistol before you even start the car.
     
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  6. alanclarkeau

    alanclarkeau Senior Member

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    As above. Firstly I'd check the for the fan under the back seat. That they're 1) not covered; and 2) clean. It's not hard to remove the vent cover of the corner vent - there's a filter inside - mine was clean, but others have reported it dirty (dog hair I think).
    upload_2019-7-12_13-36-4.png
     
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  7. alanclarkeau

    alanclarkeau Senior Member

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    The other thing - at 100 degrees, the A/C will be working fairly hard to keep you cool - but I haven't noticed that it's affected it by that much.

    Over how many tank-fulls has that 10MPG been measured?
     
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  8. kithmo

    kithmo Couch Potato

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    Disconnect the HV battery fan ? :whistle:
     
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  9. kithmo

    kithmo Couch Potato

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    This is an example of one time when having AC on increases mpg.
     
  10. CooCooCaChoo

    CooCooCaChoo Active Member

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    It might be the 25k service which involves just adding fluids and checking brakes and stuff. Could be they screwed with tire pressure. They always screw that up for me whenever I take mine in.
     
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  11. Mendel Leisk

    Mendel Leisk Sidewalk Supervisor

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    I'd like to see an A:B test on that. I monitored battery temps for a while, and saw next to no battery temperature drop, before and after turning on AC. It sounds plausible, that keeping the cabin cool would also drop the battery temp, and it's become a mantra here, but I didn't see confirmation in my (admittedly limitted) testing.
     
    #11 Mendel Leisk, Jul 13, 2019
    Last edited: Jul 13, 2019
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  12. Zardoz

    Zardoz Member

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    The 25,000 mile service. They did not say anything else.
     
  13. Zardoz

    Zardoz Member

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    Update!
    7/12/2019 - Car ran perfect all day. Saw the indicated outside temperature on the dash was 114!
    7/13/2019 - Back to problems. Here are NEW symptoms:
    1. the indicated outside temperature on the dash was 108. Parked in the sunlight for about an hour for lunch. Starting and driving away with three bars on the battery, the car was for lack of a better way to explain it SURGING as it accelerated. It felt like a car in need of a new transmission! Gas mileage was back low again.
    2. Stopped in traffic for about 5 minutes battery at 5 bars on the battery. Engine would NOT turn off so I turned off the car, waited about 20 seconds and turned the car back on. The car indicated Eco mode and the engine stayed off.
    3. Pulled off the freeway and came to a red light. Upon trying to accelerate when the light turned green, I wound up pressing the gas pedal to the floorboard and the car barely moved! No fault lights or other indicators! Eventually got it up to 15 mph with lots of surging and finally got it up to 50 for the remainder of the drive home.

    Ideas?
    Thanks!
     
    #13 Zardoz, Jul 13, 2019
    Last edited: Jul 13, 2019
  14. Zardoz

    Zardoz Member

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    I normally run the AC in Eco mode because that mode automatically set if I drive in Eco mode. I also run with the front passenger only mode on. I will try turning that off.
     
  15. alanclarkeau

    alanclarkeau Senior Member

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    I'd be going back to the dealer.
     
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  16. krmcg

    krmcg Lowered Blizzard Pearl Beauty

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    As fast as the car could go...
     
  17. Mendel Leisk

    Mendel Leisk Sidewalk Supervisor

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    That is punishing conditions for the hybrid battery, the extreme.
     
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  18. CooCooCaChoo

    CooCooCaChoo Active Member

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    Yeah, those conditions are quite extreme for the car. How do you park though? Do you leave the windows down a bit? Do you use a shade on the front windshield? Are the windows tinted?
     
  19. The Professor

    The Professor Active Member

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    So according to The Department of Earth & Climate Sciences at San Francisco State University, you can expect the temperature inside a typical car, left in direct sunlight, to rise by 34F in 30 minutes, and 43F in an hour, above the ambient temperature.

    So if it was 108F (42C) outside, the cabin temperature was probably close to 151F (66C) after that hour. The battery would have been cooler, of course, as it's in shade, lower down, and has a much greater thermal inertia than the air in the car, but still hotter than ambient as it's vented to the cabin and been warming for an hour.

    Most Lithium and NiMh based battery technology protection circuits kick in at around 113F (45C) to prevent damage by charging, and around 149F (65F) to prevent damage by discharging. So even if the battery was a few degrees hotter than ambient, the charge protection circuit was probably kicking in.

    I don't know how the charge/discharge thermal protection logic works in a Prius, but I would imagine that it would want to prevent discharging via the electric motor at around 113F (45C), due to the huge reliance on the HV system for a number of functions (e.g. starting the ICE, air con, 12V charging, etc). It will want to preserve that charge as much as it can for critical systems, if it knows it can't charge the battery.
     
  20. jerrymildred

    jerrymildred Senior Member

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    Yeah. When the cabin temperature is 140-150 as I would expect after parking in the sun, the battery is going to be pretty hot, too, and it'll run the fans to try to cool down. Problem is that running the fans just makes the battery even hotter till the cabin gets cooled down. I would fully expect the car to severely limit battery use till it gets to a safe temperature.

    Even so, this is practically a new car, so I'd still want to have the dealer check it out. I'm guessing that this is not your first Arizona summer with this car, but it is the first summer you've experienced these issues.
     
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