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Unexplained engine racing

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by BCbear, Oct 14, 2019.

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  1. BCbear

    BCbear Junior Member

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    First time post and first time Prius owner.

    Two months ago I had purchased my 2015 Prius base model with 72,000 km's on it. I have been using my car as a daily commuter as well as for photo road trips (involving some dirt/gravel roads) around my home in SW British Columbia.

    While on a road trip this weekend I was coming down a winding road on the steep side, which is normal in BC. I heard what sounded like a sudden release of air like my tire was going flat quickly. My windows were up but I thought it might be passenger side front tire but I could tell by feel/handling it wasn't the case. Within a minute or two of hearing that noise my engine started racing/revving high. I was not getting a corresponding acceleration from the high revving and I looked at my display in the various modes and saw that the engine was not showing it running, the fuel consumption meter was not showing any fuel use, the hybrid battery was near full as I was going downhill for a bit. While going down the hill I did not use 'B' mode and I was lightly on and off my brakes to control my speed.

    The engine appeared to be doing its thing completely on its own without affecting or indicating its running. At the bottom of the hill I stopped and popped the hood looking for anything unusual, steam, liquids, rushing air. Nothing. There was no dash warning lights or indications of any sort. The engine continued to rev high for a couple of minutes then slowed down for a bit then stopped. I didn't have any repeats of incident for the remaining 500 km's of my road trip going up and down countless hills, highway driving and eventual rush hour traffic in the city.
     
    #1 BCbear, Oct 14, 2019
  2. tankyuong

    tankyuong Senior Member

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    Normal due to battery overcharge prevention
     
    #2 tankyuong, Oct 15, 2019
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  3. BCbear

    BCbear Junior Member

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    Wow, what a racket!! I've been done longer and steeper roads and seen the battery indicator higher. Being a first time hybrid owner boy does the car make so many strange noises at strange times. My first time camping in the Prius the fuel evaporation test that kicked in in the middle of the night was worrisome until I could look it up in the manual.

    Thanks tankyuong.
     
    #3 BCbear, Oct 15, 2019
  4. Mendel Leisk

    Mendel Leisk Senior Member

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    If you're coming down a long/steep downhill grade, say anything over 2 kms, it's good to shift to B near the top, to help keep your speed down. You may or may not still need to brake, depending on how steep it is. If the grade flattens out enough that you need to give gas, then I would shift back to D, at least for the duration of the flat zone.

    B provides engine braking, and reduces the hybrid battery charging rate. Both these measures can delay the full charging of the battery, to the point that you're relying entirely on friction brakes.

    If the fully charged status occurs when you've reached the bottom of the hill it doesn't really matter, but if you've still got kms of steep downhill ahead of you, you might end up with overheated friction brakes.

    We occasionally need to pick up grand kids from a school that the #$%! planners located atop the steepest hill in Port Moody. Coming back down is close to 3 kms, and a drop of maybe 200 meters. For that I'll use B; it's about the lower limit.

    A more clear-cut case for B: coming down Mount Seymour...
     
    #4 Mendel Leisk, Oct 15, 2019
  5. themzlab

    themzlab Junior Member

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    Couple of things:
    1. maybe your vent hose popped off the intake manifold.
    2. need a new purge valve - it's under $100.

    this engine racing for no reason thing just happened to my 2013 in the last few weeks and I haven't proved it yet but 100% for sure that hose popped off. It is easy to find if you take off the air filter box.
     
    #5 themzlab, Oct 15, 2019
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  6. StarCaller

    StarCaller Senior Member

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    #6 StarCaller, Oct 15, 2019
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  7. BCbear

    BCbear Junior Member

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    If I haven't had the issue replicated two weeks later I assume it wasn't a popped off vent hose?
     
    #7 BCbear, Oct 23, 2019
  8. BCbear

    BCbear Junior Member

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    Can I assume it's not a popped vent hose if the problem hasn't happened again in two weeks? I finished off my trip with many more steep grades and the engine continues to purr along.
     
    #8 BCbear, Oct 23, 2019
  9. Mendel Leisk

    Mendel Leisk Senior Member

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    There's a hose connecting to the intake manifold just below the throttle body, bring gas tank fumes if I'm not mistaken.

    Is this the "vent hose" you're referring to?

    There's also the lower PCV hose. And upper PCV hose, plugs into snorkel just upstream of throttle body.
     
    #9 Mendel Leisk, Oct 23, 2019
  10. anonymous

    anonymous Member

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    If a hose popped off the intake manifold, you'd notice the engine operating extremely strangely, you'll have a lack of power, and you'll soon be forced to a stop as you're put into limp mode with the engine completely disabled (motor only until you run out of battery), "CHECK HYBRID SYSTEM" on the dash amongst other lights and a P3190 code (lack of engine power). Oh and you'd notice a whirring sound of air during engine braking.

    Since that didn't happen to you, then it's strange that there was a sound of "sudden release of air"... could it be that a hose popped off but was still aligned and got pulled back on by vacuum? But if that engine revving was caused by a vacuum leak, it should show fuel consumption on the MPG indicator... since it didn't, then it must be the system purging excess battery charge by spinning the engine with the motor, which is normal.

    Maybe you should check for any loose hoses on the intake manifold and EVAP line just in case. They're under the air filter box.
     
    #10 anonymous, Oct 24, 2019
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