Leaky Head Gasket and Prior Complete Coolant Exchange - "Non-Hybrid"

Discussion in 'Gen 2 Prius Main Forum' started by Bechi, Jul 25, 2025 at 12:01 PM.

  1. Bechi

    Bechi Junior Member

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    This is still troubling me, so thought I'd see if anyone had any thoughts.

    About two weeks ago my 2007 touring with ~340K miles beeps and red triangle appears briefly with thermostat icon. We pulled over, could find no issues and overflow tank is full. No other symptoms so I figured "Princess" was getting pissy in her golden years at having my young man drive her. Later, checked for codes and found none, but saw level in radiator was low so added asian pre-mix coolant, and let her run with full heat until radiator would take no more - about a quart total. Took her out next day for a long ride while young man monitoring temps using techstream and averaged 180°F to 190°F. Seemed all was well.

    A few days later when I was driving to work, same thing happened - beep, red triangle, and thermostat icon. When this happened, I noticed that the heating system was acting strange and I wasn't getting heat. I brought her in to the nearest stealership and they were not able to pull any codes, but did notice flakes of dried coolant on the bottom of the engine bay, but no evidence of a leak or any coolant loss. The technician though it was possible that it could have been due to a coolant splash that got pulled through the fan when my local stealership replaced the coolant two months ago.

    Since they didn't have an answer, I asked them to replace the thermostat since it seemed like a reasonable place to start being as it had never been replaced. When replacing the thermostat, the technician noted that there was evidence of a leaky head gasket and it could last for "a while" or go at any time. Princess still runs fine; no shudder, no white smoke, and no sludgy oil. I have not had to add any coolant since picking her up at the stealership a week ago.

    I have been casually looking at new models for a few years now, but with this diagnosis, plus an intermittent combi meter issue, and the original traction battery I decided now was the time and have a Gen5 AWD Limited reserved. This is still bugging me though, because according to all reports, Gen2 head gasket issues just aren't at all common, I am not only the original owner of this vehicle, but ordered it from the factory, so can say with all certainty that she has always been maintained (between my father who was a mechanic for almost 70 years, my quasi-mechanic motorhead young man, and the local stealership). I also read back through the service record for the coolant replacement done in May, and it's listed as "Complete Coolant System Fluid Exchange in both Engine and Radiator. Non-Hybrid". Why would it be listed as non-hybrid? I don't know of any reason why changing the coolant in a hybrid would be any different than changing it in an ICE-only vehicle, but that just seems weird.

    I'm curious if anyone has any thoughts about this. Thanks,

    Bechi
     
  2. dolj

    dolj Senior Member

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    My interpretation of this phrasing is that they only changed the engine coolant and not the inverter coolant, so they serviced the non-hybrid portion of the system.
     
  3. rjparker

    rjparker Tu Humilde Sirviente

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    It was losing a lot of coolant and likely had air pockets that fooled you when adding coolant. Gen2 has more air pocket issues.

    A better explanation and picture of the evidence would have been my request but it sounds like it was leaking outside of the engine if he saw something. Gen3 hg fails leak into the cylinder and foul the plugs causing misfires and shutters.

    I agree with the above, a complete "hybrid" flush is two separate cooling systems, not just the engine.
     
  4. Bechi

    Bechi Junior Member

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    Thank you, both! That makes more sense to me. I'm pretty sure he said he did put a borescope into the cylinder and saw evidence of coolant leaking in, but after pretty much spending the day in the waiting room losing my sanity, I could be misremembering. There was someone who insisted on singing while they waited for quite some time...

    When I have taken it out in the past week, I've been running it with full heat just to purge any lingering bubbles and just topped off the radiator to the top of the neck in the event that not only was the system not fully bled during the May coolant change, but also during the thermostat replacement. I'll continue that and monitor it to see how much it's going down from here.
     
  5. rjparker

    rjparker Tu Humilde Sirviente

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    With a coolant leak into a cylinder, combustion gases can be pushed into the cooling system replacing coolant burned off in a cylinder. A positive borescope test is the gold standard for head gasket leaks.
     
  6. Bechi

    Bechi Junior Member

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    I did notice a bit of a "whoosh" when I opened the radiator cap to check and add more coolant. I'll continue to monitor it (for scientific purposes natch), but with all reports of a leaky head gasket on a Gen2 being so rare, and usually accompanied with codes, white smoke, shuddering, etc. I'm just curious (hopeful?) that maybe it was just the thermostat and any bubbles may be getting bled out. If that's the case, it should become apparently pretty quickly. If it IS the head gasket, I've duly reported that it isn't so rare after all, and isn't necessarily accompanied by the obvious symptoms.

    Thanks!