Another prius with mileage issue...

Discussion in 'Gen 2 Prius Fuel Economy' started by Ynprius, Nov 13, 2018.

  1. Ynprius

    Ynprius New Member

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    Sorry to do this to the forum but yes, I am posting a thread about my low MPG on my 09 Prius.

    My 09 Prius has 86K miles and I normally got 48-50 mpg in all seasons. About two months ago (sometime mid-September), I noticed that my MPG was starting to get gradually lower. At first, I thought maybe I was driving a little too aggressively and had been hurting my MPG cause of it so I paid no mind to it. But even after driving more conservatively, my MPG kept getting lower (over weeks). Eventually, I thought "maybe my tires are a little deflated" or "my oil needed changing" (it was about that time). So I attended to both but it didn't help my MPG. Now I'm getting 36 MPG and I can't for the life of me figure out why? The only thing I've noticed that's different is that my battery doesn't seem to hold its charge as well-- I think. I'm an avid observer of my energy consumption and what I usually saw was the hybrid battery in the green like 85% of the time. Now it's in the blue or half charged 85% of the time (exaggerating a bit but you get my point). Is there potentially something wrong with the hybrid battery?

    Any thoughts/advice is appreciated.

    Thanks!
     
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  2. bisco

    bisco cookie crumbler

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    welcome!

    1) no worries

    2) are you measuring tank mpg at the pump?

    3) generally, a healthy prius battery is in the mid range (blue) not upper (green)

    4) have you checked the air and cabin filters? 12 volt health?
     
  3. Ynprius

    Ynprius New Member

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    Im measuring MPG based of what the info screen tells me which I had found to be accurate. They checked air/cabin filters and 12v health during oil change and it was fine.

    Interesting, my prius charge was always more in the green before. Not fully charged. Always with 1 less bar from max charge but still usually green.

    One thing I noticed last night is that my gas tank is filling up less then it ever was before. The tank in my 09 prius touring should be 11.9 gal but I never got close to that when I first got the car. Before, even after having to roll the car into a gas station one time, the tank would only fill max 10.5 gal. Now the last two refills have maxed at ~8 gal. Any thoughts as to why?
     
  4. bisco

    bisco cookie crumbler

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    there is a rubber bladder inside the tank that contracts with the cold. when did you buy it?
     
  5. bisco

    bisco cookie crumbler

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    what is your typical driving like, and about how many miles a day?
     
  6. 05PreeUs

    05PreeUs Senior Member

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    The change in HV pack behavior does suggest that is a factor.

    MPG is miles driven divided by fuel used, anything the electronics on the vehicle does is a GUESS AT BEST. You cannot reliably use "lie-o-meter" readings to gauge much of anything.

    You suggest "all seasons", how long have you owned the car; ie: Was it reporting better MPG last winter than now?
     
  7. Ynprius

    Ynprius New Member

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    Last fall. Didnt have any issues like this.

    Conservative. I drive mininum 30 miles a day.

    Yea. Had the car since last fall. During the winter, my MPG was usually high. Only in the summer did it decrease a little and that was only cause of AC and the lowest was 46 mpg.
     
  8. Kevin_Denver

    Kevin_Denver Active Member

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    Check your 12V battery. If it's dying the Prius computer will constantly try to charge it from the HV battery and you'll see your charge levels dropping more quickly than normal. Also winter driving will cost you fuel efficiency as well.
     
  9. bisco

    bisco cookie crumbler

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    idk, not many people here get the same mpg in all seasons.
     
  10. Ynprius

    Ynprius New Member

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    UPDATE: My fuel economy dropped to about ~33 MPG so I decided to take it into a local mechanic for a quick checkup. Mechanic ran some diagnostic tests and found nothing out of the ordinary (ie no error codes). He didn't check the condition of the HV battery (said I would have to leave overnight) but said the 12V was fine based off the diagnostic. He recommended maybe new spark plugs and cleaning the throttle body. I think the car still has the original spark plugs but I haven't experienced any misfiring, stuttering, etc usually associated with bad spark plugs. Should I still change them? The mechanic didnt actually check the throttle body but said that sometimes hurts FE.

    Interestingly enough, after leaving the shop, I filled up and reset my Prius graph and now my fuel economy is at 42 MPG. Not my usual MPG but getting closer... Maybe somehow the diagnostic test fixed something???

    Still only filling up ~8 gallons as opposed to my usual 11 gallons. I found this thread with very similar issue so probably really is a rubber bladder issue after all but not sure why it started happening all of a sudden. Is there a way to replace it or something?

    Also, now the MPG gauge seems to drop after starting up the car. For example, I would start up the car and within 2-3 minutes, the MPG goes from 42 to 40. Then I have to drive super conservatively (aka let the car glide a lot) to bring it back up to 42 MPG. Any thoughts as to why it does that initial drop?
     
  11. bisco

    bisco cookie crumbler

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    plugs don't get changed until 120k, they are not likely the issue.

    throttle body is worth checking

    can't change the bladder, and it only affects tank size, not mpg.

    you can't trust the gauge, put a couple hundred miles on her and check at the pump

    have we talked tires?
     
  12. Ynprius

    Ynprius New Member

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    I'll try and check the throttle body this week. Tires are good. I'll see how the fuel economy is in the next two weeks or so (usually how long it takes to empty tank).
     
  13. hamilhead

    hamilhead Junior Member

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    I am having similar issues with 2007 Prius - gradually dropping mileage. When I first got it at 70K miles, it regularly clocked 45-47 mpg (I calculate by miles/gallons per tank. Now its around 40-42 mpg on a good tank... I drive 10K /year or less .
    Have about 129K miles on it, live in NorCal (where we think its really cold when it gets in the 40s) and my driving is pretty consistent. A typical week = 8-10 trips of 3.5-5 miles driven at 25 mpg steady; All other trips on freeway between 10-50 miles, with 2 miles drive at 25 mph to get on/ off freeway. I'm gentle on gas/brakes, gliding to red lights and hoping to not come to full stop, easing into acceleration as best i can (short freeway ramp doesnt allow for slow run up) I consistently drive 25-30 mph local and 65-67 on the freeway. Change oil at 5K. buy 87 gas. keep tires at around 35 front/rear, per my mechanic's suggestion (they know a lot about Prius)
    is it just age? what else can I do? might be needing new tires soon...
     
  14. bisco

    bisco cookie crumbler

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    have you kept up with regular maintenance, including the major service of fluids and spark plugs?

    put the car on a lift and make sure none of the brakes are dragging

    test the 12 volt health

    you might need fuel injector service
     
  15. Scott Rundell

    Scott Rundell New Member

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    I'm having a similar issue,(2004 with about 77000) After replacing the 12v battery and having the oil changed i noticed a significant drop in MPG going from about 47 mixed driving to about 43 mixed, Any ideas?
     
  16. bisco

    bisco cookie crumbler

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    check the oil level

    test the 12v

    test the hybrid battery

    are you calculating mpg's at each fill up?
     
  17. Scott Rundell

    Scott Rundell New Member

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    Tested the 12v with the signals test in the car 12.4 under load and 14.4 charging

    oil level is about quarter quart high

    going off the numbers from the trip computer, i will test the old fashion way next fill up. that begs the question with the bladder how do i know it's really full ?

    i have no way to check the traction battery, i ordered a bluetooth odb and dr. hybrid when they arrive i will test. Anything else i can look at?
     
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  18. bisco

    bisco cookie crumbler

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    unfortunately, with the bladder, all you can do realistically is keep long term averages and look for trends.

    who changed the oil? i'm thinking they messed with your average by running the 12v down, and the car having to recharge it

    check your tires pressure, air and cabin filters
     
  19. Scott Rundell

    Scott Rundell New Member

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    Air filter replaced at oil change, Oilstop (a drive in oil change place similar to jiffy lube). Why the cabin filter out of curiosity ?
     
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