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2008 Prius MPG Issues

Discussion in 'Gen 2 Prius Main Forum' started by Gunnar Boag, Nov 18, 2021.

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  1. Gunnar Boag

    Gunnar Boag Junior Member

    Joined:
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    Vehicle:
    2008 Prius
    Model:
    N/A
    Hello everyone!

    I am a new Prius owner as of six months ago. I am slowly trying to learn about this hybrid vehicle, but so far it's been a piece of cake. Everything was going great until my catalytic converter was stolen in October. My local shop quickly replaced it and put a guard over it, so problem solved right?

    Well, about two weeks ago I noticed my mpg's took a serious dip. I'm talking 25-30 mpg currently (I had been getting 43-44 before that). I've been trying to trouble shoot this issue and I can't seem to find a good solution. The details:

    2008 Prius with 136,000, bought used. I'm in Oregon so it is getting colder, but I can't imagine the mpg takes that much of a dip in the winter.

    The hybrid battery seems to be working as it had been the past 6 months. It charges well and sits at six bars regularly.

    Since the CAT replacement, there has been an occasional grinding noise when I accelerate after being stopped. I'm wondering if this could be because of a lame CAT that my shop put in...

    I recently changed my oil and the air filter. There are no error lights on the dash. I checked the 12 v battery and it is at around 12.24v after a night of sitting. I've read a bad 12v could cause problems, but I would be surprised if it changed THAT much within the Prius.

    I'm sort of at my wits end with this one. It feels like nothing is really horribly wrong with the car, yet it is getting horrible mileage. It all started suddenly after the CAT replacement, but could that really be the issue? Or is there another problem that started up coincidentally? Should I take it back in and have them potentially swap for a new CAT? Any help is much appreciated!
     
    #1 Gunnar Boag, Nov 18, 2021
  2. nancytheprius

    nancytheprius Active Member

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    Location:
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    Vehicle:
    2006 Prius
    Model:
    II
    Welcome! The cold has an impact, but not that drastic to where you would see 25-30 mpg. (Don't think you should ever see that unless you're driving like 90 mph). Is your driving more then 5 or 10 minutes at a time? The first five minutes on the display will show that low from the warm up cycles, but I am guessing your drives are longer. I have read on other forums that some cheap aftermarket catalytic converters can cause the drop, and seeing as it times perfectly with your situation that would be a likely cause. You seemed to be thorough with your troubleshooting, so I will let someone else give their input.
     
    #2 nancytheprius, Nov 18, 2021
  3. nancytheprius

    nancytheprius Active Member

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    I did an advanced search with "catalytic converter mpg" The first one had the same drop in mpg upon replacing the cat with an aftermarket. Said it was fixed by cleaning the mass air flow sensor. Maybe try that out. maf Cleaner is like 6 or 7 dollars and its an easy part to get to.
     
    #3 nancytheprius, Nov 18, 2021
  4. Another

    Another Senior Member

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    Vehicle:
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    Model:
    Touring
    Have the 12v checked under load at parts store. They’ll do it for free. Takes two minutes.

    What brand and model of catalytic converter did you get? If the guy tells you he doesn’t know, that’s a red flag.

    Get a OBD Bluetooth code reader and see if any codes have been triggered. short trips are a two prong problem in colder weather, they don’t charge a cold 12v battery and the gas engine or ICE runs longer.

    You can also use the free Dr Prius app to check HV battery.
     
    #4 Another, Nov 18, 2021
  5. The Lizard King

    The Lizard King Member

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    You need a better aftermarket catalytic converter, and that grinding noise (it's probably a rattle) is your aftermarket catalytic converter vibrating against your new shield.

    Cheap catalytic converters aren't always cheap in the long run (poor gas mileage). I bought a fairly pricey one:



    And my mileage is still 45-50 mpg.

    As far as the rattle goes, I would get new rubber exhaust hangers, which should raise your catalytic converter away from your shield.

    Rubber Exhaust Hangers
     
    #5 The Lizard King, Nov 18, 2021
  6. Gunnar Boag

    Gunnar Boag Junior Member

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    Vehicle:
    2008 Prius
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    I'm going to take it back in and have them check the converter. Something is telling me that is the issue here. When they are done with it I will probably clean the MAF sensor as I have some of the cleaner at home already!
     
    #6 Gunnar Boag, Nov 18, 2021
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