2008 Weird MPG...need help

Discussion in 'Gen 2 Prius Fuel Economy' started by arkana, Feb 16, 2012.

  1. arkana

    arkana New Member

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    Hi all,

    I'm new here but have done a fair bit of reading. Hoping for your input on this as I try to figure out what the problem is with my car.

    To sum it up my last 5 tanks have gone something like this 48 mpg, 38 mpg(got me thinking Wth), 42 mpg, 36 mpg, 49 mpg, and now 27 mpg.

    There are almost zero variables in weather, driving habits, tire pressure, etc.

    The only things I can think of are the 12 volt has died a few times b/c I left a light on. My key fab light flashes at me on the dash on random occasions. And the gas tank gauge is crazy off sometimes and spot on the other half. The mileage gauge in the car has been reading below 40 mpg on all of the above tanks.


    - 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)

    Using manual calculations. Miles traveled divided by number of gallons pumped.

    - What fuel economy are you expecting and why?

    48. I averaged that for 20,000 miles on this car.

    - What are the approximate outside air temps?

    Lately around 55-70 degrees.

    - How long are your trips?

    Varied but on some all commute tanks(under 10 mile trips) I've averaged 48 mpg and on some of the same style it's been 36.(and now 27).

    Same bizarreness on 100% highway miles tanks.

    - 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?

    See above. I don't go crazy fast. Not a huge amount of stop and go even in the city driving. On the highway it's between 65-75.

    As mentioned my driving pattern is the same and never had a problem previously. I always averaged between 44-50 depending on these types of conditions.


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

    Arkansas

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

    Half the state is flat and half hilly. Just depends. On some hilly highway trips I've gotten 52 mpg. And when this weirdness started I've gotten 36 going on flat interstate the whole way.

    - Is your oil overfilled? (i.e. above the full mark on the dipstick)

    No.

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

    I guess it's the original one. I haven't checked the voltage reading. Still trying to figure out how that would affect mileage. Wanted to get you all's opinion first before I take it in(not sure how to check the voltage).

    Have you had your alignment checked? Any pulling or abnormal tire wear?

    It's fine currently.

    - 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).

    They are Coopers. I need to run out and check the brand(dark here right now). They are the same size as the factory ones I believe.

    - What are your tire pressures?

    35 all around.

    - 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.)

    I drove an 8 cylinder GMC Sierra 1500 truck which I still have. I got about 15-18 mpg on it.

    - How are you trying to drive (e.g. trying to stay in electric only?) and how hard are you braking?

    I'm not braking hard that I know of. I slow down using the 'b' button and try to conserve momentum.

    - Are you "warming up" the ICE (internal combustion engine) by letting it idle after powering on?

    Sometimes.

    - 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?

    Haven't used the a/c or the heater in a while.

    - 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?


    As mentioned the 12 volt going dead a few times is the only thing that has been weird. That and the gas gauges being way off.

    One time it signaled low fuel and I put 4 gallons in it and got 48 mpg. Then the next time it signaled low fuel, I almost didn't believe it. It was accurate this time and took almost 10 gallons.(and awful mpg).

    Thanks in advance for the help!
     
  2. KK6PD

    KK6PD _ . _ . / _ _ . _

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  3. bisco

    bisco cookie crumbler

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    i vote new battery. running it down can be detrimental to its health.
     
  4. F8L

    F8L Protecting Habitat & AG Lands

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    The erratic nature of the problem leads me to the 12v battery as well.

    Also, the Cooper tires are not likely LRR so that will cost you some mpg.
     
  5. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    Some ways that the 12 volt battery fails fools the inverter into charging it continously. This is power not going into better mileage and it never stops (while READY) until you replace the battery.

    A 'low tech' way to check is first thing in the morning press Power twice without your foot on the brake then try the power windows. now push Power with your foot on the brake and see if the windows are much faster
     
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  6. Codyroo

    Codyroo Senior Member

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    Your fuel tank bladder will be causing some of the problems with fill ups. But the overall randomness of it points to something else. The 12 volt battery is small and from anecdotal stories, can't take many if any discharges like leaving the lights on without permanent damage to the 12 volt battery.

    Does anyone else drive the car (maybe reset your trip odometer, took the car on a road trip when the parents were out of town)?

    Any chance the car was left in "Ready" mode overnight and the engine was turning off and on all night long?
     
  7. arkana

    arkana New Member

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    Well your low tech way worked perfectly. They were much slower prior to putting my foot on the brake and doing a full power up. Guess that means I need to order a new battery for sure....Thanks for the tid bit!
     
  8. arkana

    arkana New Member

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    They are CS4 Touring tires. The owner of a major tire shop where I buy has them on his Prius. I've gotten 48+ out of them. Thinking if I put some more PSI in them and fix the other problem(now appearing to be the battery) they should be fine.
     
  9. arkana

    arkana New Member

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    Do you guys recommend this battery to replace right?

    Optima 8071-167 D51 Yellow Top Dual Purpose batter
     
  10. ryogajyc

    ryogajyc Active Member

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    No, the direct replacement is DS46B24R. You can check on Optima's website by searching by vehicle. This place is selling it for $170:
    http://www.elearnaid.com/opdsopbadifi.html
     
  11. arkana

    arkana New Member

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    Hey guys, just wanted to report back about this. I bought a new battery and had it installed at the Toyota dealership.

    I also inflated the tires to around 40 psi. Unfortunately this last tank only got 33 MPG. That's pretty bad.

    Weather was 70 degrees for the whole tank. I'm not really sure what's going on. I got 48 MPG on these same tires when they were 36 psi when I first got them.

    I haven't changed my driving habits or anything. For the first year I owned this thing every tank was between 45-50.

    The new battery doesn't seem to be doing much(although the windows roll down quicker:).

    Any help is appreciated. I'm needing to get back in the 40's on MPG at least.
     
  12. arkana

    arkana New Member

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    Bump, anybody got any ideas on what the problem could be?
     
  13. uart

    uart Senior Member

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    The erratic measurements are probably due to the tank bladder and inconsistent fill volumes.

    The real fuel consumption is probably whatever is the "below 40 MPG" reading that your multifunction display is reporting.
     
  14. arkana

    arkana New Member

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    Ok, thanks. Well my display is showing an abysmal 37 mpg. Why would I see a 20% drop in fuel economy for no apparent reason?

    Even during the winter months when I first bought the Prius(bought used) I was getting a consistent 45+, sometimes 52 MPG. It was ridiculously consistent for the first 20,000 miles I put on it.(bought it with 40,000, now has 65,000+).

    Thanks again for the help!
     
  15. uart

    uart Senior Member

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    Yeah I agree that the drop in MPG is strange, I just wanted to point out that worrying about inconsistencies from per tank calculated MPG's is likely to just confuse the real issue.

    Since you've just replaced the battery, give it a few tankfuls to re-learn the "historical data" that it uses to fine tune the displayed MPG. Also, you might try doing a longer term record of miles driven and gallons used (again over several tankfuls) to try and minimize the tank to tank re-fill variations.
     
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  16. arkana

    arkana New Member

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    Ok, thanks, seems like I'd almost need to run it dry and then calculate over several tanks.

    If I can't rely on assuming that it's truly full after a fill up makes it tough to calculate. Either way I'll give it a go over the next 600 miles or so.

    Any other things that could be causing this? The only other thing I can think of was the very first time I noticed it I came on this forum and read about the oil being overfilled.

    Sure enough they had filled it above the line. Ever since then, and even after draining some oil(and numerous other changes) I've been getting odd mileage levels.

    Any chance the main battery is going out? Thanks again for the help.
     
  17. KK6PD

    KK6PD _ . _ . / _ _ . _

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    Do you run "Auto" on the Heater A/C and have the temp set to something like 80° or so? Doing something like that will cause the ICE to run much more than should just to keep the cabin temp were it's set. Have you taken the car to a Dealer to see if any codes were thrown? What is the condition of the MAF sensor? Does that need cleaning. Unless you are really in love with those tires, they gotta go, look for some good LRR replacements, F8L has a thread on this, it's worth looking at.
     
  18. arkana

    arkana New Member

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    Hey, no I don't run the heat or AC on anything ridiculous. The weather has been 72 for the last month anyway so haven't even used it lately.

    I guess I need to take it to a dealer to get checked out. As mentioned with the current tires and everything else I was getting a consistent 48 MPG's so something different is happening.

    Will definitely look into some LRR's in the future, these tires didn't hurt the mileage though. Thanks for the help, will check into the sensor issue and see what the dealer says. Thanks!
     
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