2019 Prius Slow Coolant Leak

Discussion in 'Gen 4 Prius Care, Maintenance and Troubleshooting' started by JeffPGH, Jun 25, 2026.

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  1. rjparker

    rjparker Tu Humilde Sirviente

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    No. It might cost a little mpg. Toyota design it to open on high exhaust pressure alone when high rpms are being used.

    There is no single root cause of gen3 head gasket failures unless you want to blame the engine design and testing team when it was adapted from a Corolla for use in a hybrid. It worked fine in a conventional Corolla which even had the egr system. The Corolla also had low tension rings which lead to excessive oil burning as did Camry and Rav4s but Corollas did not blow head gaskets without overheating.

    It's clear Toyota quickly knew they had problems using the Corolla engine in a thermal cycling hybrid with an Atkinson cycle camshaft. Five years later a redesigned 1.8L solved the hg problems for gen4 which is why many have embraced using parts of a gen4 in a gen3 car, eg a Frankengine. Hybrid Pit now sells a rebuilt gen4 block, a modified head gasket and a gen3 head topped with a gen4 cam carrier. But most older gen3s are not worth using their most expensive option when fully rebuilt and moderately improved gen3 engines are available for less.
     
    #41 rjparker, Jul 2, 2026 at 7:06 PM
    Last edited: Jul 2, 2026 at 7:45 PM
  2. JeffPGH

    JeffPGH New Member

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    I put the UV Dye in and drove around for a while. I took a look as best I could without being able to lift it into the air. I didn’t see any obvious signs of leaking. That doesn’t seem like a good sign.
     
    #42 JeffPGH, Jul 4, 2026 at 5:00 AM
  3. bisco

    bisco cookie crumbler

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    could the new cat be leaking?
     
    #43 bisco, Jul 4, 2026 at 1:57 PM
  4. JeffPGH

    JeffPGH New Member

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    I was wrong on my last message and I tried to erase it but could only edit it. I thought it drained overnight but it is actually pretty close to full. It was hard to see with the UV Dye in it until I put a flashlight on it. Might be the new cat. Not sure. Probably gonna need to take out the spark plugs and see if any dye went in there.


    Coolant Leak Tracking
    • 6/26/26 Put on new cap
    • 6/27/26 344542 Coolant was a little below the Low line. Topped it off to Full after it sat all night.
    • 6/28/26 344768 (226 miles) Coolant was about 1.5 inches below full line…. It was below the low line. Topped it up to full
    • 6/29/26 344934 (166 miles) Coolant was right at the Low line. Topped it up to full.
    • 6/30/26 345168 (234 miles) coolant was halfway between the Full and Low line. Topped it up to the Full line.
    • 7/01/26 345414 (246 miles) coolant was almost at the bottom of the reservoir. Put the Prius in maintenance mode to keep the engine running and did the block test. It stayed blue. I put the tester by the exhaust pipe and confirmed that it started turning green. Let it cool down and topped it up to the Full line.
    • 7/02/26 345624 (210 miles). Coolant was down near the bottom of the reservoir again. Topped it off to the full line. Planning on putting UV Dye in it tomorrow.
    • 7/03/26 345800 (176 miles). Coolant at the bottom of reservoir again. Put in UV dye and topped off to full.
    • 7/04/26 345981 (181 miles). Coolant near the full line.
     

    Attached Files:

    #44 JeffPGH, Jul 4, 2026 at 2:01 PM
    bisco likes this.
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