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Sudden drop in MPG...?

Discussion in 'Gen II Prius Care, Maintenance and Troubleshooting' started by tvspaulmoore, Jul 17, 2013.

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  1. tvspaulmoore

    tvspaulmoore Junior Member

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    Hi there

    Our 2007 Prius had previously been averaging high 40s MPG but the past few weeks has dropped to around 38 and won't move much higher... we are using the A/C a little more than the past few months but not on full constantly or anything like that... first time I've seen MPG this low since we've owned the car (20 months)

    Does anyone know what could be causing this? Would only a Toyota dealer be able to rectify?

    We have about 77K on the clock.

    Thanks!
  2. SteveLee

    SteveLee Active Member

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    With any electrical, MPG, or other weird issues, always suspect the 12V battery until that possibility is eliminated. Get the voltage readings with the process here. Weird stuff happening? MPGs dropping? Test The Battery | PriusChat

    You can just use the parking lights instead of the headlights. Posting the values back here will help others diagnose the problem.
    Sheepdog likes this.
  3. tvspaulmoore

    tvspaulmoore Junior Member

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    Thanks, I will check tonight and post the results.
  4. nh7o

    nh7o Off grid since 1980

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  5. tvspaulmoore

    tvspaulmoore Junior Member

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    Thanks for the info and links

    I have filled in the questionnaire but the headlines are that our tires are overinflated- 40PSI front and rear and the battery seems on the low side of voltage- not sure how much difference that would make (details below)... and also not sure if battery is at the stage it needs replacing...?

    Would testing the battery first thing in the morning before driving yield any more useful information? Apologies if these questions are all obvious- this is our first Prius (an 07 we bought in 2011) and the first time I've had a car have a sudden double-digit drop in MPG...




  6. SteveLee

    SteveLee Active Member

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    What brand and weight of oil did you use?

    Some like the higher tire pressure as it reduces tire surface on the ground and therefore resistance. I personally prefer 38-36 as the higher pressure is results in a harsher ride and I have seen little if any difference in MPG. But the factory recommended pressure is to soft a ride. The type of tire, especially being a low rolling resistance tire, does make a significant difference but that doesn't seem to be the issue in this case with your sudden drop in MPG.

    The battery readings are not strong but not that bad. It is charging good. Yes, checking again in the morning before starting would be good. There is still no guarantee this battery is not the culprit. But checking battery cable connections is not a bad idea. The negative connection to the body has been known to come loose.

    Does the engine cycle on and off as before, like turning off at a stop light, and running about a mile below 42 mph for about a mile before the engine kicks back on?

    In town driving is not the best mpg scenario but assuming that hasn't changed from before, not the issue.

    Regular driving and braking like any other car is not the best method for Prius mpg. Relearning to drive anticipating stops far ahead to take advantage of regenerative slowing/braking captures already expended (and paid for) energy back to the battery reduces the need for the engine to charge, thereby increasing mpg. Basically, learn to drive by the MFD, the energy graphic readout. But again, this probably hasn't changed, though it can improve the result.

    Also, anytime you leave the car in a shop, there is the chance the guys will leave the car in "ACC" mode to run the radio. This can be death to a battery as the little 12V does not have much stamina. Draining it once can be all it takes and it doesn't take very long. Anytime accessories are run the car should be in the Ready mode so the ICE will kick on and charge the battery as needed.

    I'm still leaning towards thinking the 12V is the culprit. Let's see what it shows in the morning.
  7. wjtracy

    wjtracy Senior Member

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    Lots of us changed over to Optima Yellow Top batts. You could check to see if you have 12v Toyota batt in the back. I am changed my 2006 12v out Jul_2011 after about 5-yrs so if your batt is orig you are past 5-yrs.

    PS- the tires are not familiar to me as the ones people here tend to get
  8. mfa-prius

    mfa-prius Old member

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    I posted a few years ago that my '05 seems to be bipolar -- mileage is either 50ish or 40ish, with lifetime average of 47.57, and 5-fillup averages of from 36.4 to 55.8. I've never been able to correlate this behavior with much of anything. Recently it was running in low mode, so I replaced the 7-year-old 12V battery with a yellow-top, figuring that it was time, even though it checked out OK-but-weak. Didn't make any difference. Would a failing traction-battery cell produce the same effect? Just over the last few days it seems to be back in high mode, with the tank average now climbing above 47 after being around 43 for the 1st half tank. I have the feeling that it declerates too quickly when I back off the accelerator -- certainly slows more rapidly than does my wife's '12 v -- but there's no other indication of brake drag.

    Here's the questionnaire:

    - Have you read This Thead Yet?
    Yes

    - What fuel economy are you getting and how are you determining fuel economy? (trip computer or manual calculations)
    I posted a few years ago that my '05 seems to be bipolar -- short-term mileage is either 50ish or 40ish, with lifetime average of 47.57, and 5-fillup averages of from 36.4 to 55.8. Graph of any measure shows a steady decline from new until now.

    - What fuel economy are you expecting and why?
    Expecting 48, hoping for 50. My driving style is conservative, I live in a flat state, temperature is moderate-to-high.

    - What are the approximate outside air temps?
    Today, expecting 73°F-90°F.

    - How long are your trips?
    Typically 30 miles.

    - How much of it is city vs. highway? Roughly what's the average speed in overall and and of each segment? Is there a lot of stop and go driving?
    Mostly rural, 45-50MPH, with occasional 10-mile highway spurts of 70MPH, infrequent 50-mile highwat trips. Essentially no stop-and-go.

    - What region/state are you in? (if you haven't set your location in your profile)
    Central FL.

    - What's the terrain like of your drives? (e.g. flat, gentle hills, steep hills, etc.)
    Flat

    - Is your oil overfilled? (i.e. above the full mark on the dipstick)
    No. Using Mobil1 5W/30.

    How old is your 12v battery? What is the voltage reading of your 12v battery after sitting over night? (Method Here)
    12.5V

    Have you had your alignment checked? Any pulling or abnormal tire wear?
    Yes; no.

    - Are you using the factory tires and wheels? If not, please indicate tire make, model and size (e.g. Goodyear Assurance Fuel Max 185/65R15).
    Factory wheels; Yokohama AS530 185/65R15, foisted off on me by the dealer. Worst tires I've ever used anywhere, but I had the bipolar problem since new.

    - What are your tire pressures?
    38F, 36R

    - Make, model, year, engine and transmission of previous car? (e.g. 08 Honda Civic Si 2.0L 4 cylinder, manual transmission) What did you actually get on the same trips/commute? (Please give us actual numbers, not EPA ratings.)
    2002 Chevy Silverado 1500, V8 Automatic. 17.32MPG lifetime.

    - How are you trying to drive (e.g. trying to stay in electric only?) and how hard are you braking?
    Driving conservatively, but not a fanatic about it. Seldom use friction braking, drives my wife nuts. My test track is a 7-mile stretch of flat road approaching home. Driving at a steady 41MPH indicated, with the car warmed up, I see over 50MPG in "good" mode, under 45MPG in "bad" mode.

    - Are you "warming up" the ICE (internal combustion engine) by letting it idle after powering on?
    No.

    - Are you driving using D or B mode?
    D.

    - HVAC settings? Are you using the heater, AC, auto mode, etc.? If using auto, what temp is it set to?
    AC Auto, 76°F.

    - If reporting a mileage drop, did anything significant change on your car (e.g. accident, hit a curb or big pothole throwing off alignment, oil change/other maintenance/repairs, changed tires or wheels, etc.) or your commute?
    No. Short-term mileage drops and rises over multiple-fillup periods with no obvious correlation with anything I can think of. On a particularly hot day, I experimented with turning the AC on and off while driving at a steady 45MPH on my flat-road test track, and saw no difference in MPG reported in the MFD.
  9. Rich12

    Rich12 Member

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    Tire PSI should be 42/41.
  10. tvspaulmoore

    tvspaulmoore Junior Member

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    The battery reading this morning after not being used for 12+ hours gave 12.2 (unload)/11.9(load)/14.3(car idle- engine did not kick in, but in 'ready' mode).

    Battery cables did not appear to be loose but to be completely honest I couldn't figure out which was the negative- is that the one which attaches to the rear? If so, it looked pretty solid.

    The engine seems to cycle on and off as before- driving doesn't seem different but I will drive it again and see (my wife drives it so I haven't used it today).

    The oil (according to the receipt from our auto shop) was Valvoline- no more details given. They changed the oil filter too.

    The only other random element I can think of is that it has rained pretty heavily here lately- our car sits in the driveway uncovered- but I don't know whether rain could penetrate anything which would cause a drop in MPG...

    Thanks to anyone who can suggest anything... it's such a drop in MPG I can't bear to simply drive around and not know what's going on... but as mentioned above, am reluctant to drop $$$ for Toyota to have nothing to look for if they try and find an error code.



  11. wjtracy

    wjtracy Senior Member

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    Hi MFA, I have also reported this behavior. We have same 47 MPG life as you do. I have noticed we can be going along at say 55 MPG on highway, stop fop for gas, now we are reading 45 MPG on the MFD. I tend to blame it on in-car MFD computer reading which I tend to ignor and go by the hand calc from actual gallons. Generally speaking the lower reading seems closer to hand calc.
  12. SteveLee

    SteveLee Active Member

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    Things like filters and plugs come to mind but the mileage is exceptionally high. You may want to think about getting a scan tool that will get a readout of the HV battery cells. Lower mpg must be from the engine working harder and/or less efficiently for some reason. Weak cells in the HV I believe will cause the engine to work harder to turn the MG to recharge the HV.

    Did they by chance adjust the brakes or emergency brake? Are all the coolant levels up? Any strange noises from the engine area, maybe a coolant pump?
  13. tvspaulmoore

    tvspaulmoore Junior Member

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    Thanks for the reply. Does the 12V sound OK from the measurements I got?

    I will drive again and listen out for strange noises but nothing exceptional stands out.

    What sort of scan tool should I look for? I've never owned/used one but have seen them... will they all read the HV cells?

    Lastly- is there a resource anywhere for recommended hybrid auto shops? I'm not expecting one here in FL near me but worth asking.

    Also... is it possible the MFD readings are way off? We go a while (or used to) between fillups so would be a while before I could manually compute (and I never used to manually measure so have nothing to compare other than previous MFD MPG averages)
  14. SteveLee

    SteveLee Active Member

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    I meant to say. NOT exceptionally high.

    The battery readings look ok.

    I don't have a scan tool but a search here in PC will bring up some good threads about them. They are not all equal.

    For hybrid shops in Florida you can also do a search here. I suspect there will be one not too far away.

    Manually measuring mpg for the Prius, due to the bladder that fluctuates in how much fuel it allows from one fill-up to another, is by the gallons pumped and miles driven calculated over several tanks.

    I think the MFD is generally accurate within 2 mpg, assuming correct wheel and tire size.

    I suppose you are using the same source for gasoline?
  15. uart

    uart Senior Member

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    To be honest, if there's no codes logged then Toyota probably couldn't rectify the problem either. That's been the experience of most people who've posted here with declining MPG issues.




    Yes, the 12V readings look fairly typical for an aging but still functional battery. It's unlikely to be the issue in my opinion.



    The wrong oil grade will have an effect on MPG, but very unlikely to the extent you are talking about (more like 1% to 4% rather than 20%).

    One other possible factor, and indirectly related to the oil change, is that some shops disconnect the 12V negative any time they do even the simplest job on a Prius (concern about engine starting at random). When the 12V battery is disconnected, certain "historical data" that the Prius uses to calibrate the MPG calculations is lost. Typically this causes the displayed MPG to become inaccurate, in some cases over estimating and in some cases under estimating the true MPG. There is a small possibility that your actual MPG hasn't changed, only the readout. If this is the case then it would correct itself over a period of weeks or months (depending on how much you drive).

    A final possibility is that they initially overfilled the oil and then noticed the error and drained some, perhaps *after* it ran and stalled. This causes oil to get into the throttle body, so I'd try running a bottle of any good brand injector cleaner in a tank of fuel. I know some people think it's snake oil, but it's easy enough to do and I think it's worth a shot in cases like this where no other obvious reason for reduced MPG can be found.
  16. Rich12

    Rich12 Member

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    Our 2007 Prius had previously been averaging high 40s MPG but the past few weeks has dropped to around 38 and won't move much higher... we are using the A/C a little more than the past few months but not on full constantly or anything like that... first time I've seen MPG this low since we've owned the car (20 months) Does anyone know what could be causing this? Would only a Toyota dealer be able to rectify? We have about 77K on the clock. Thanks!


    *sigh* So you are one of those "nancy boys" who seem to think it's okay to use the A/C because "it's so hot out and my little pink panties can't take it but I don't want my fuel economy to suffer."

    Look, kid, I did Aviation Officer Candidate School under the loving care of Staff Sergeant L.E. Wills, USMC, forty (yes, count 'em!) years ago. In my class 33 started and only 11 were left to get wings. AND NO WE DIDN'T HAVE A/C.

    If you can't take a little heat and back off the A/C then you deserve to have the MPGs take a hit.

    Pussy.
  17. tvspaulmoore

    tvspaulmoore Junior Member

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    Thanks. I will try this. Otherwise I guess I will wait and see if it ticks back up over a few tanks. Never had this before from an oil change at the same place though.






    Hi maybe I am having sense of humor failure and this reply is unnecessary but... the MPGs are worse than when I drove in similar circumstances with A/C running as intensively as last summer so I am effectively comparing like with like.
    Dino33ca and Prius_Cub like this.
  18. nh7o

    nh7o Off grid since 1980

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    A/C is important in hot weather. Use it, as that is what keeps the HV battery cool. HV battery life and car performance is dependent on cool batteries. Letting them get hot repeatedly is false economy. And normally the MPG hit with A/C on is small.
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  19. mfa-prius

    mfa-prius Old member

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    Keeps me cool also, which is every bit as important :). And my imprecise testing shows negligible hit with AC on or off at 41MPH on a flat road at 90°F ambient.
  20. vskid3

    vskid3 Active Member

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    A Scangauge II might be able to read them, but I haven't done much research into the sgauges available for the Prius. I do know that a bluetooth scanner with the Torque app for Android can read the battery block voltages, not sure how useful it would actually be for diagnosing a dying battery. I definitely recommend it if you already have an Android phone or tablet. See this thread for more info.

    I don't think AC (except maybe on MAX COLD) makes too big of a difference in "normal" or cruise control driving, especially above 40MPH. Where I notice a difference is when pulse and gliding. There just isn't enough charging time to keep the HV battery charged. I usually have the AC set to at least 78. I monitor the HV battery temp and the fan speed, the fan doesn't spin up fast enough even with the battery at 115 for me to think turning up the AC would help.
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