2019 Prius Slow Coolant Leak

Discussion in 'Gen 4 Prius Care, Maintenance and Troubleshooting' started by JeffPGH, Jun 25, 2026 at 3:51 PM.

  1. JeffPGH

    JeffPGH New Member

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    A couple years ago my Prius was leaking coolant. I took it to the dealership and paid them for diagnostics and they said I needed a $1500 Catalytic Converter. My buddy said that was a known issue and he did the bypass for me. A couple months ago my coolant started leaking again. I found out about the TSB from September 2019 (T-SB-0135-19) on the NHTSA government website about the coolant leak issue and called Toyota Corporate and complained and managed to get them to replace the pipe under warranty even though it was over the mileage for the warranty. I thought that would fix the issue but later on that day the coolant reservoir was empty. I put some coolant in and took it in the next morning. The looked it over and said everything was fine and there was probably air in the line. However, after I left I had to put coolant in around every 4-5 days. I took it back and they did a pressure test and they said it passed. They tried swapping the caps between the two different coolant reservoirs. It was fine for about a week then I had to put coolant in again. I ordered a new coolant cap just to try it to see if it might help and they said it would be here tomorrow. I had them change the spark plugs when they had it the other day for the coolant pressure test and they sent me a photo of them. It did look like one of them was a lot whiter than the others but I mentioned it and they didn’t seem concerned. One suggestion I saw online was to get an inexpensive block tester kit (chemical fluid test) to check the coolant reservoir for the presence of exhaust gases. I haven’t tried that yet. I’m not getting any check engine codes or smoke coming out of the exhaust. They said since they replaced the entire heat exchanger pipe with the new part in the TSB that probably isn’t the issue. Since there aren’t any check engine codes I could probably just keep running it and checking the coolant to make sure it isn’t low but I’d rather find the problem and solve it if possible. I did notice when I use the funnel to pour in coolant it seems to leak out the top a little bit by the plastic piece that sticks out with the little black end on it. I think that’s the overflow but I’m not sure. I might not be using the correct funnel.

    Does anyone have any ideas on what else to check?
    IMG_9957.jpeg IMG_0032.jpeg
     
    #1 JeffPGH, Jun 25, 2026 at 3:51 PM
    Last edited: Jun 25, 2026 at 3:56 PM
  2. bisco

    bisco cookie crumbler

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    is that dried coolant all over everything?
     
    #2 bisco, Jun 25, 2026 at 9:24 PM
  3. Brian1954

    Brian1954 Senior Member

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    It looks like dried coolant all over the plastic reservoir. To troubleshoot, you need to monitor the engine coolant temperature. Watching the coolant temperature will tell you if the coolant is getting hotter than it should be.

    My theory is the engine water pump is going bad. The pump is not pumping enough coolant through the engine to keep it cool. The coolant heats up too much and creates enough pressure in the system that some coolant is released past the reservoir cap and out that little overflow nipple. This is common behavior for the Gen 3 Prius when the pump is going bad. I have not seen many reports for the Gen 4. Try putting a rag around the reservoir overflow to see if that is where the coolant is being released from.
     
    #3 Brian1954, Jun 25, 2026 at 11:50 PM
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  4. BiomedO1

    BiomedO1 Senior Member

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    ^ Yep; coolant splash in that area would indicate that it's boiling out. Kinda obvious to any experienced mechanic. Assuming the dealership did the pressure test correctly, (that's a BIG if) - since he should've noticed that "right off" - it's probably the electric ENGINE water pump not spinning fast enough to keep the coolant from boiling out - but fast enough not to trip the temp lamp at 248+F. Swapping coolant reservoir caps? They should've been inspected and a missing sealing ring would cause leaks and not allow the system to properly pressurize.
    You can still have a head gasket issue; if that leak test was performed with the engine warm or hot. The aluminum would expand and seal the leak - that's why we say to pay attention to early cold morning startup misfires that disappears after a few minutes.

    Good Luck.....
     
    #4 BiomedO1, Jun 26, 2026 at 11:35 AM
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