The more I replace, the worse mpg gets. WTH?

Discussion in 'Gen 2 Prius Fuel Economy' started by GTW, May 26, 2020.

  1. GTW

    GTW Junior Member

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    Good evening. Before I get started, I will state my typical mpg was anywhere from 39 - 42. US. I am religious with oil changes every 5k miles, and I run with tire pressures 42/40. When I purchased the car I replaced the spark plugs, air filter, cleaned the MAS and throttle body. Lastly, the HV pack is in average health. No failing modules according to Dr. Prius, my state of charge is usually in the 40-50% range.

    Then I started repairs.

    Recently I posted here about replacing front wheel hubs with product from Rock Auto. I had a local shop install them. I was expecting my mileage would adjust a little bit as these hubs get settled in, but the mileage got worse(now down to 35-36.

    The grinding noise of the old hubs was gone, I now could hear my wheels were 'roaring'. Off I go to tire shop, and they showed me where one rim was noticeably bent, and another was slightly out of round. Okay, I decide to purchase new rims and tires all around. Again, tire noise was gone, but now I notice a slight, slow oscillation coming from the back that was masked before. The oscillation slowly alternates from side-to-side, but It doesn't seem to affect straight-line steering. Mileage goes down again. Now 29-30. With new hubs, rims, and wheels, I still have to get an alignment done.

    I have been running with a P0420 for a couple of months. I have read that a Prius can drive with that Cat code for a long time, so I wasn't sweating it. I'm going to do a tune-up, as soon as it stops raining. The Prius still drives just fine, with no noticeable change in the way it handles, or accelerates. There are no new codes other than the P0420. Could the Cat be so bad that my mileage has gone down the drain?.

    I am hoping the tune-up solves some of the mystery. Any thoughts? Thank you.
     
    #1 GTW, May 26, 2020
    Last edited: May 26, 2020
  2. Leadfoot J. McCoalroller

    Leadfoot J. McCoalroller Senior Member

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    Well if you think about it a burning house can usually stay standing for a while after the smoke detector starts beeping. That doesn't always mean it's a great idea to keep living there.

    Somebody needs to do real diagnostic work to figure out if this problem is centered in the catalyst or if the problem is further upstream. If the engine isn't running correctly it could be creating a legitimate catalyst efficiency problem.

    Given that you've done ignition and airway work recently it may be worth revisiting those just to make certain. Both of those departments can contribute to misfires, misfires can foul the cat and it's off to the races from there.
     
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  3. Raytheeagle

    Raytheeagle Senior Member

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    Have you checked your brakes recently or are they hot after a drive?
     
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  4. TMR-JWAP

    TMR-JWAP Senior Member

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    Spend about 350 bucks and buy a complete new cat assembly from the exhaust manifold flange to the flange at the rear wheels. Install it and see what happens. Take the original one and sell the 2 Toyota cats on ebay for 850-900 bucks. You make 500 bucks and have a new cat.
     
  5. Mendel Leisk

    Mendel Leisk Sunday driver DIY’r

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    If you're running elevated tire pressures on rough roads, that might be contributing to wheel bearing failure. Toyota recommendation is 35/33 psi (front/rear).

    How many miles on the car btw?
     
  6. SFO

    SFO Senior Member

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    How many miles on her, and is that Castroville in California?

    Check those plugs again, could be fakes if not purchased from a local retailer.

    What does your average commute/trip look like in time/distance, etc.
     
  7. Leadfoot J. McCoalroller

    Leadfoot J. McCoalroller Senior Member

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    ^ yup, lots of counterfeit plugs on ebay & amazon lately. Probably others too.
     
  8. GTW

    GTW Junior Member

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    Good advice. The weather cleared long enough for me to do a partial tune-up. I rechecked my throttle body and cleaned it. It was showing just a little bit of oil in the bottom but not as much as when I had first purchase it. Secondly, while I was there I sprayed the MAS again. Third, being in the same area I removed, inspected, and cleaned the VVTI and filter.

    IMG_20200526_194450.jpg

    The screen looked good, so I just sprayed it down with brake parts cleaner, let it air out, and put it back in.

    That was all I had time for before the rains started up again.

    Here is the curious part. The next day, a short trip across town displayed its typical 40 miles per gallon. I went and filled up the tank, reset the mileage, reset the consumption, and drove a 50 mile round trip. Suddenly the fuel economy is back to 'typical'.

    I can't explain the sudden loss in mileage, nor can I explain its sudden return. I didn't really change anything except clean the VVTI.

    The next thing on my to-do list is to address the catalytic converter. I'm just not sure what parts are correct for an 07.

    Thanks for your input, I appreciate your observations.
     
  9. GTW

    GTW Junior Member

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    I have been running 42/40 on my tires from mostly highway driving, I have hit a couple of potholes that had me worried. With new front-wheel hub assemblies, rims, and tires, I'm hoping all I should need is a final alignment.

    My spark plugs are straight from Toyota.

    Raytheeagle made a good observation. The shop which installed the hubs should have put the brakes back together correctly. It's the number-2 item on my list after the Cat. Thanks.
     
  10. GTW

    GTW Junior Member

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    Do you have a recommendation for a specific cat? eBay search for 2007 Prius turned up 100s of results, from dangerously cheap to pretty expensive. I see a couple of Walker cats, but how do I know if they are counterfeit or not? I don't have a whole lot of experience buying 'new' through eBay.
     
  11. Leadfoot J. McCoalroller

    Leadfoot J. McCoalroller Senior Member

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    Look before you leap.

    P0420 ≠ bad catalyst.

    It might be a bad catalyst, but really that code means diagnostics are required so you can work out whether it's a leaky injector, undetected oil burn, flaky throttle positioning motor, bad wire on a sensor etc etc etc.

    I don't have the specific workup for that code on your car, but maybe somebody else has it handy.
     
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  12. SFO

    SFO Senior Member

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    None of the application specific or direct fit options can be legally shipped or installed within CA.

    You might find this post/thread helpful. Installing a non CARB compliant catalytic converter could cause you to fail the CA smog (visual) test.
     
    #12 SFO, May 28, 2020
    Last edited: May 28, 2020
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  13. Bob-W8RH

    Bob-W8RH New Member

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    Sounds like what happened to me with my 06 Gray 297,000. The cat was clogging up. It finally failed and the internals broke apart. Ran fine once I took it off and emptied out the melted ceramics !!!! Yes it still threw a code, but the mileage went back to normal.
     
  14. ice9

    ice9 Active Member

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    First off, your 39-42 mpg was a little low at the start. I consistently got around 45 mpg with my gen 2. When I purchased it back in 2009, I had heard that the Prius was notorious for wheel alignment issues and you had to keep an eye it, but I never had a problem with this. If you didn't purchase your gen 2 new, you may have inherited a lemon.
     
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  15. GTW

    GTW Junior Member

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    I had Toyota address the alignment, and also got an estimate for the Cat. $$$$
     
  16. ice9

    ice9 Active Member

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    ...or maybe a bad cat, as mentioned above... ...I hadn't thought of that. Did you get a chance to check that out? I'd hate to see you drop the dollars, only to find out that the cat wasn't the problem.

    Before you take it out to inspect tho, I would look into having your cat cleaned by someone who does that kind of work, or turning it in to getting a cleaned/overhauled replacement. There are a number of threads here regarding this subj. The problem, of course, is having to wait for it. Or perhaps, if you are brave, you could try cleaning it yourself (see highlighted link to PC article).
     
  17. ice9

    ice9 Active Member

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    Belay my last. I was confusing a contaminated catalytic converter with clogged egr issues. Completely different problem. I wouldn't think that a contaminated converter could cause the low MPG numbers you are experiencing.

    If you really need to replace your cat tho, you still might want to look into having you catalytic converter cleaned instead of replaced.
     
    #17 ice9, Jun 5, 2020
    Last edited: Jun 5, 2020
  18. GTW

    GTW Junior Member

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    I had a Subaru that was subjected to a catalytic converter cleaning and it was completely ineffective. Anyway, Toyota told me the oxygen sensors are good, but the cat needs replacing ($2,800). I do appreciate the link. I might just give that a try.
     
  19. NinnJinn

    NinnJinn Member

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    Are you sure you're state of charge is usually in the 40 to 50% range??

    In my gen 1 Gen 2 and gen 3 if state of charge dropped in the low fifties it was doing everything possible to bring up the state of charge.

    all three Pri that I've had the car was usually between 58 to 64% state of charge very very rarely have I ever seen at 55% or below..
     
  20. GTW

    GTW Junior Member

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    Fairly sure. I usually only have two blue bars, 3 if I get on a slight descent.
     
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