2019- heat exchanger done- cap popped off

Discussion in 'Gen 4 Prius Care, Maintenance and Troubleshooting' started by C Clay, May 5, 2026 at 8:43 PM.

  1. C Clay

    C Clay Member

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    This car is new to me and has 200,000 miles on it. It just started the issue with the coolant loss and the heat exchanger.

    I did the bypass underneath the car, that seemed pretty easy. I read that the port to the heat exchanger should be capped off?

    Well, I did that, but I didn’t secure the caps and one of the caps popped off under pressure. Should I have vacuumed or tried to blow out all of the coolant from the exchanger? It’s actually in the catalytic converter isn’t it? Do you just have to wait for it all to burn off and then cap it off? I don’t remember reading this.
     
    #1 C Clay, May 5, 2026 at 8:43 PM
  2. ChapmanF

    ChapmanF Senior Member

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    It's in the exhaust pipe, behind both catalytic converters.

    (In gen 4, the first catalytic converter is built into the exhaust manifold. So the exhaust pipe starts with the second catalytic converter, followed by the heat exchanger.)
     
    #2 ChapmanF, May 5, 2026 at 8:45 PM
  3. C Clay

    C Clay Member

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    Just asking a hypothetical question- if the cap popped off- would that imply that the system is sealed, ie- that there is no failure in the heat exchanger?
     
    #3 C Clay, May 6, 2026 at 9:04 AM
  4. C Clay

    C Clay Member

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    I’m trying to figure out the pros and cons of this device. What is the effect of bypassing? Just that one might struggle for heat?
     
    #4 C Clay, May 7, 2026 at 7:39 PM
  5. ChapmanF

    ChapmanF Senior Member

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    Engine takes a bit longer to warm up.
     
    #5 ChapmanF, May 7, 2026 at 8:35 PM
  6. rjparker

    rjparker Tu Humilde Sirviente

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    If the exhaust heat exchanger failed there is now an exhaust leak into your "capped" coolant lines. I would probably securely cap it.

    You should also ensure the thermostatic valve is secure in an open position to reduce exhaust restriction.
    IMG_1383.jpeg
     
    #6 rjparker, May 7, 2026 at 9:04 PM
    Last edited: May 7, 2026 at 9:14 PM
  7. C Clay

    C Clay Member

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    Thank you, I thought I saw you post a picture in another thread on how you did this?
     
    #7 C Clay, May 9, 2026 at 4:55 AM
  8. BiomedO1

    BiomedO1 Senior Member

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    You've got your answer in this thread....
    Looking for help doing the heat exchanger bypass | PriusChat

    Your more than likely creating another future problem by bypassing a perfectly good EGHE, and pretending to tell us you have a leak. Think about it - if it's leaking, why did it blow off the cap??? Wouldn't the excess pressure just leak into the exhaust system? :(:cry::whistle: Path of least resistance. That's why problems needs to be tested, rather than going by assumptions alone....
     
    #8 BiomedO1, May 9, 2026 at 5:36 AM
    Last edited: May 9, 2026 at 12:10 PM
  9. rjparker

    rjparker Tu Humilde Sirviente

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    Assuming your bypass fixed the coolant loss, cause and bypass effect was determined.

    A 2019 would take longer than a 2016 to experience the issue if it was going to happen.

    Another member forced the exhaust heat exchanger valve into the straight through open position here:
    Need help with Custom PIDs for Gen 3 Electric Water Pump (21D7) in Torque Pro | PriusChat
    That was on a gen3 so your gen4 could be slightly different.

    To find a recent post from a member click on their name and then "recent posts". One issue with Priuschat is the post title may have nothing to do with the solution. This is because the op was guessing at first about the problem and edit ability is quickly locked out for admin paranoia reasons. In the link above the member started looking for custom scanner pids to diagnose a water pump but it was a stuck exhaust heat exchanger valve.
     
    #9 rjparker, May 9, 2026 at 10:03 AM
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