This is Why Stop Leak Should be Banned

Discussion in 'Gen 3 Prius Main Forum' started by jimolson, May 16, 2026 at 12:18 PM.

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  1. jimolson

    jimolson Member

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    My brother and I work on Gen 3 Priuses acquired from disenchanted owners. The seller of this one said the vehicle "runs just fine...no problems".

    Of course it rattled like a spray paint can with a blown head gasket, as most Gen 3's purchased from FB Marketplace do.

    Replaced the motor with a different one rebuilt by the esteemed Scotty Partin, a trained Toyota hybrid tech in Rockholds, KY.

    But after installing Scotty's rebuilt motor the vehicle still overheated on up-sloped highways, despite having a new thermostat and water pump in it.

    The problem was revealed to be the stop leak additive poured in the cooling system by vehicle's former owner. It plugged the bottom half of the radiator.

    My thanks to Scotty for his help on the project. He would not have known about the stop leak problem because he never saw the vehicle or its blown engine.

    Surprisingly the vehicle's heater still functions like a blast furnace, though I imagine that its water passages are pretty gunked up.

    BTW, Gen 3 dashboard's engine overtemp icon illuminates at 250 degrees F. But a cheap OBDII tester running in the front seat can help you stay below 210F by driving the vehicle slowly home on a cold day.

    I cringed that Toyota set the temp threshold so high to turn on the overtemp light.
     

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    #1 jimolson, May 16, 2026 at 12:18 PM
    Last edited: May 16, 2026 at 12:27 PM
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  2. Tombukt2

    Tombukt2 Senior Member

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    Yeah that's very common it'll even INFlate up out filler neck . Really wild looking stuff usually. Pulling rad and presxur flushing all else . Sad
     
  3. PriusCamper

    PriusCamper Senior Member

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    In other words, if you don't know vehicle history of a gen3 and you're swapping or rebuilding the engine you need to decide it you're replacing the radiator or not...

    I wonder if there's a flow rate test to determine if radiator is viable? Anyone have an suggestions?
     
  4. bisco

    bisco cookie crumbler

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    generally, by the time the over heating light comes on, the engine is blown
     
  5. rjparker

    rjparker Tu Humilde Sirviente

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    It is very likely the flow into the reservoir tank will be higher when the radiator is partially clogged. This flow can be observed and could be measured.

    Radiator and reservoir flow starts when the thermostat opens. Before then, only flow through the thermostat bypass port to the engine occurs.

    IMG_1460.jpeg
    The reservoir flow parallels the radiator but does not go through the cooling channels of the radiator.

    The flow restriction caused by the radiator micro channels creates pressure differential driving flow into and out of the reservoir.
     
  6. Mendel Leisk

    Mendel Leisk MMX GEN III

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    One stop-leak manufacturer recommended a flush with 50/50 white vinegar and water, might help.

    I totally agree: stop-leak products should be banned. They're ostensibly to eke a few more miles out of a head gasket, but a ploy too easy to use when selling to unsuspecting buyers. :mad:

    Hey, maybe Toyota should market a stop-leak product, specifically for 3rd gens... :whistle:
     
  7. PriusCamper

    PriusCamper Senior Member

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    Has anyone found a way to modify their Prius so you get a warning at a lower temperature?
     
  8. bisco

    bisco cookie crumbler

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    not that I recall, but as o/p mentions, a cheap old reader works fine
     
  9. rjparker

    rjparker Tu Humilde Sirviente

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    Sometimes I think you just unblocked me
    Prius P10 Scanner.jpeg

    $35 P10 with user set audible alarm on temp. Probably posted 15 times here.
     
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  10. BiomedO1

    BiomedO1 Senior Member

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    Use an IR temperature gun to shoot different areas of the radiator. Deviations should be small, clogged area deviations are larger. This is a lot faster than pulling the radiator to flow test it.
    You can also pull the lower radiator hose to inspect for jelly like gunk...... There should be NO contamination.
     
  11. AzWxGuy

    AzWxGuy Weather Guy

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    I added an IR imaging camera to my toolset a while back. Just a small little thing that plugs into my phone. I don't think I'll ever need it to show radiator blockage, but I did test it on my warmed-up car. The HVAC condensor is first in line, viewing through the hood latch access port. With A/C and both radiator fans running, the temperature was very uniform across the condensor. I imagine reduced airflow due to obstructions (bugs, etc.) would appear warmer. It would be difficult to image the ICE radiator though, since it is behind the HVAC radiator.