1. Attachments are working again! Check out this thread for more details and to report any other bugs.

Removing coolant cap

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by Wtfdustinwtf, Feb 20, 2024.

  1. Wtfdustinwtf

    Wtfdustinwtf Junior Member

    Joined:
    Jul 5, 2019
    27
    12
    0
    Location:
    Carolinas
    Vehicle:
    2010 Prius
    Model:
    One
    Hello, I remember reading here in various threads about a test for knowing if there is a compromise head gasket is to remove the coolant reservoir cap and sound will be heard. I can't seam to figure out if the "sound" is a hissing or a suction. From my understanding a hissing, pressure coming out, is normal, however if it is a suction(pressure going in) then that's the sign of a bad HG. Can someone confirm this? I recently removed my cap and there was a hissing, in which air and tiny splatter of coolant came out of the tank.
     
    #1 Wtfdustinwtf, Feb 20, 2024
  2. ChapmanF

    ChapmanF Senior Member

    Joined:
    Mar 30, 2008
    23,765
    15,409
    0
    Location:
    Indiana, USA
    Vehicle:
    2010 Prius
    Model:
    IV
    Depends on when you open it. If the engine was running recently, the coolant is hot and there will be pressure.

    If everything is cold, there should not be much pressure difference either way. If there is an exhaust-to-coolant leak at the head gasket, the system may still be pressurized with exhaust. If there is a leak of coolant out, there may be a negative pressure. (Many coolant caps have a relief valve for negative pressure too, though. I've never looked really closely at this one to see.)
     
    #2 ChapmanF, Feb 20, 2024
  3. PriusCamper

    PriusCamper Senior Member

    Joined:
    Mar 3, 2012
    11,055
    4,498
    0
    Location:
    Pacific Northwest, USA
    Vehicle:
    2007 Prius
    Model:
    Two
    Like Chapman said, sometimes based on changes in temperature a small amount of air will blow out, or small amount of air will be sucked in, even in a healthy Prius...

    A failed headgasket on the other hand will lose significant coolant so the sucking in sound will be much more extreme. But in general, the easiest way to know HG failure is both engine running rough periodically and how much coolant you have to add.
     
    #3 PriusCamper, Feb 20, 2024
(You must log in or sign up to post here.)