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2010 Prius ICE wouldn't shut off today

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by EfficiencyNerd, Dec 13, 2016.

  1. EfficiencyNerd

    EfficiencyNerd Junior Member

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    Forgive me if this has been posted before, but I couldn't find anyone with the same situation. Long time lurker, first time poster....

    I drive a 2010 Prius with 146k miles.

    Today as I was just getting to work, I noticed the engine turn on for some unknown reason as I got off the highway. There was no one behind me, so I was deliberately using the regen the entire way, so initially I had two thoughts - 1) I had charged the battery enough that the engine started to burn it off, or 2) something finally happened with my spark plugs, which have yet to be replaced (yes, I know... that needs to happen real soon). After this happened as I drove the last mile or so to work, the engine never shut off. The car had been running for about 45 minutes at that point, mostly on the highway, so the engine being cold is definitely not the cause.

    Then on the drive home, the engine turned on immediately and didn't shut off for the entire drive home, even when coming to a crawl or stop. The battery for pretty much the entire drive home sat in it's "default" state (or what I presume is a default state of sorts), or about 2-3 bars below full. Just as I pulled into my apartment building's parking garage, I decided, hey, I wonder if hitting the EV button will give me some error code or something. But instead... the engine actually shut off. Using the electric engine worked just fine. When I hit the EV button again, the engine immediately turned on. I drove around the parking garage enough to determine that every time I put it into EV mode, the engine would promptly shut off, and when taken out of EV, the engine immediately turned on and wouldn't shut off.

    What the heck is going on?

    About the spark plugs - I haven't noticed anything weird about them yet, still getting decent mileage (just shy of 50 MPG, so not bad for winter tires and below freezing temps), and it still seems to run fine and has no shortage of power. It does grumble a little when starting up and it's really cold out, but nothing other than that.

    Reading some forums, perhaps it could be the 12V battery? I know it has been changed once already, but I'd have to dig out the log to figure out when that was.

    Thanks. Any thoughts or suggestions are appreciated.
     
  2. JC91006

    JC91006 Senior Member

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    If your heater is on, the engine won't be shutting down much
     
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  3. bisco

    bisco cookie crumbler

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    welcome! 'deliberately using the regen'? worth checking the 12v, hope it's not your hybrid battery.
     
  4. working1

    working1 Active Member

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    Not shutting off could be a symptom the 12V battery needs to be replaced. ICE will keep running in an attempt to charge a bad 12V battery.

    Worth checking out before getting stranded in cold weather.

    Warning signs the 12V battery is on life support | PriusChat
     
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  5. Former Member 68813

    Former Member 68813 Senior Member

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    +1 especially in canada.
     
  6. FrankB

    FrankB Member

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    The ICE does not charge the 12V battery as far as I know, it's charged by the inverter from the high voltage battery. The ICE does not have any 12V charging device like an alternator.

    Anecdotally, when my 12V battery was going bad, my ICE never ran all the time but my mileage seemed to decline towards the end due what I assume is the ICE charging the high voltage battery so it could charge the 12V battery.
     
  7. fotomoto

    fotomoto Senior Member

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    How long have you owned it?
     
  8. Kramah313

    Kramah313 Active Member

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    Could be a sign of the HV battery going bad? Have you noticed if the battery gauge goes from empty to full and back again really quickly? On a freeway drive with reduced capacity it may have filled it up really quickly and just started running he engine. Or the engine is trying to protect it from some other sensor reading it might be giving off.
     
  9. EfficiencyNerd

    EfficiencyNerd Junior Member

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    Hi all, thanks for the replies. In response:

    I don't run my heater... yes I live in Canada, but I just wear a hat and gloves. I don't need a climate controlled environment everywhere I go, plus I'm a nerd and would rather suffer in the cold to get better MPG lol.

    By 'deliberately using the regen' I probably should have said 'deliberately not using the brake pads'. I know I'm always using the regen brakes when braking (except under ~7mph). I was deliberately not braking hard enough to engage the pads in order to re-coop as much charge as possible.

    It could have been something with the 12V battery... although I'm not noticing much of a noticeable drop in MPG.

    No signs of the HV battery going dead. Seems to increase/decrease at the normal rate.

    I fail to see how long I've owned the car as being a factor in why my ICE engine wouldn't shut off, but if you must know, I have owned the car about a year now, since it had 130k on it. I have the full service records as they were done by the dealership previously, and the seller gave them to me. I also pulled a carfax when I bought it which matched the records given to me. The car was taken *very* good care of.

    At any rate, I still have no idea what the issue was, however, today the ICE was turning on and off just as it's supposed to. Once it had warmed up in both morning and evening drives, it was turning off properly when not accelerating hard or at highway speeds. I was able to accelerate at the normal rate on just the electric engine while in traffic, so it seems to be operating perfectly now.

    I'm not sure if just the car fully cooling down overnight (in a 55-60 F heated parking garage) reset it or if me hitting the EV button last night influenced it in some way. I'll pay closer attention to the engine over the next few days, but for right now everything seems to be fine. My best guess is the engine turned on to burn off excess charge, and for some reason didn't turn off. Perhaps the charge was still too high when I first got in the car in the evening, but that doesn't explain why it never turned off for the whole drive home (until I hit the EV button in the parking garage).
     
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  10. rrg

    rrg Active Member

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    If the 12v is OEM on a 2010 then I recommend to change it.
    If it was changed once then you should be good on that part.
    I just did my battery on a 2011.
     
  11. Mendel Leisk

    Mendel Leisk Senior Member

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    I'll speculate your engine is not shutting off because the car's computer deems the engine is not warmed up enough. A couple of reasons I can think of:

    1. Extra cold weather, coupled with higher than normal cabin heat requests, say coupled with setting vent mode to heat/defog. I believe you say you're driving with coat and mitts, keeping cabin temps low, that's likely ruled out, lol.

    2. A failing engine coolant thermostat. This is the first thing that came to mind, when you mentioned highway drive, then when pulling off highway engine continued to run. Highway runs tend to cool extra efficiently due to high speed wind through the radiator. If the engine coolant thermostat is failing in the open position, the coolant will cool too efficiently.

    Any econobox car these days will have a coolant temp gauge. They're invaluable for showing engine warm-up status. Well, except for Toyota Prius. :rolleyes:

    Anyway, this is something to check out, if you can get a hold of a ScanGuage or other means to monitor coolant temp. Or dealership could troubleshoot.

    With 146K miles, deterioration of the thermostat is a possibility, and considering that it's not too expensive, worth trying replacement, just a "low hanging fruit" trial. If the problem persists.

    If you do opt to dive in and pull the thermostat yourself, I would suggest to just buy a new one, have it on hand before starting. If you're going to the hassle of removing the existing one for checking, might as well replace it. When doing the test, suspending thermostat in a pot of water on stove with thermometer, suspend both the old and new thermostat, compare performance.

    There's a couple of ways the existing thermostat could be going south, as likely as not both will be occurring:

    1. There's a rubber gasket around the whole thing, that seals, prevents the coolant from flowing into the radiator until it's warmed. Over time and use the gasket gets brittle and starts to fray, allowing coolant to constantly leak by, regardless of the thermostat being closed.

    2. There's an internal, temperature-regulated mechanism that opens the thermostat. Over time it too can deteriorate, so that the thermostat opens too soon, before coolant temp has risen sufficiently. This is observable by testing the two thermostats side by side in a pot of water, as you raise the temperature.
     

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    #11 Mendel Leisk, Dec 22, 2016
    Last edited: Dec 22, 2016
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