Overheat and coolant loss from reservoir, how to tell if water pump or head gasket

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by QuinnVee, Jul 7, 2025 at 7:23 PM.

Tags:
  1. QuinnVee

    QuinnVee New Member

    Joined:
    Yesterday
    1
    0
    0
    Location:
    Tucson, Az
    Vehicle:
    2012 Prius
    Model:
    Three
    Hi there!

    A few weeks ago my 2012 prius (105k miles) overheated, while highway driving for a few hours in the 110 heat. I slowed down and the overheat light went away but the check engine light was on. The codes were po117 and p148f and there was spilled coolant on the reservoir. I refilled the coolant and it hasn't noticeably lowered but I have found marks in my driveway. I've had to drive it around town the last few weeks and it hasn't thrown anymore codes or overheated.

    I brought it to the dealership to see what they'd say and they recommended replacing the water pump and thermostat because of the codes and the spilled coolant around the reservoir.

    I'm planning on replacing the pump and thermostat myself ( and maybe cleaning the egr while I'm in there). Before I do so I want to find out if there are head gasket problems. What should I be looking for/ what tests should I be running. Torque shows no misfires on start up and the oil on the dipstick looks regular to me. I am fairly new to car stuff so any advice is wanted.

    Thanks!
     
    #1 QuinnVee, Jul 7, 2025 at 7:23 PM
  2. NutzAboutBolts

    NutzAboutBolts Senior Member

    Joined:
    Mar 5, 2013
    1,599
    1,684
    0
    Location:
    Bay Area
    Vehicle:
    2012 Prius
    Model:
    Four
    If it’s your headgasket, you can remove the spark plugs and put a borescope down there to see if the pistons top is wet or not. Also, you should have a leak in the coolant reservoir if it’s the HG
     
    #2 NutzAboutBolts, Jul 8, 2025 at 3:37 AM
  3. T1 Terry

    T1 Terry Active Member

    Joined:
    Jan 4, 2015
    730
    397
    0
    Vehicle:
    2006 Prius
    Model:
    Plug-in Advanced
    Get a gas test on the coolant, if there is combustion gasses in the coolant, it's a head gasket, if not, put cardboard down under the engine and check in the morning just where the coolant leak is dripping from .... you might need to remove the plastic tray under the engine (if it still has one) and redo the drip test a few times to isolate where the problem leak is .... the water pump leak will appear at the fan belt end of the motor, easy enough to replace, unlike the gen 2, Google Search
     
    #3 T1 Terry, Jul 8, 2025 at 4:15 AM
  4. ASRDogman

    ASRDogman Senior Member

    Joined:
    May 29, 2018
    7,213
    3,772
    0
    Location:
    Florida
    Vehicle:
    2010 Prius
    Model:
    Two
    That test doesn't work on the Prius because it's the coolant that leaks into the cylinders.
    Unless it's REALLY bad, but by then you won't need that test, you'll KNOW it's the head gasket
    and likely a warped head.

    There is no pulley on the Prius coolant pump.

     
    #4 ASRDogman, Jul 8, 2025 at 7:18 AM
    Brian1954 likes this.
  5. Brian1954

    Brian1954 Senior Member

    Joined:
    Jan 31, 2021
    1,779
    693
    0
    Location:
    South Central PA, USA
    Vehicle:
    2011 Prius
    Model:
    III
    I agree with the dealership that you need to replace the engine water pump before you drive the car again. An OEM pump is preferred. Do not buy a cheap aftermarket pump on Amazon or eBay. They do not last very long, and you will end up having to replace it again.

    If you have a bad head gasket, you would get misfires (rattling) during cold startup of the engine. I do not think that you presently have a head gasket issue. The P0117 code indicates that the engine severely overheated. This can lead to the head gasket going bad 6 to 12 months from now.

    So, replace the water pump ASAP and consider a replacement car in 6 to 12 months if you do not want to deal with a bad head gasket.
     
    #5 Brian1954, Jul 8, 2025 at 9:32 AM
(You must log in or sign up to post here.)