I have changed my radiator cap couple of months ago with an original OEM one. The older one had a broken sealant which caused engine over heating.

Now, i have no heating problems but after the engine cools down for 4-8 hours I notice the upper radiator hose gets flatten!

I watched a YouTube video - he was telling the symptoms of a bad radiator cap, in the middle he said "That sucked radiator hose is a good sign of a well sealed coolant circuit!"

So do I have to consider changing the hose or there is another problem to worry about?

Thank you,

  • 1
    I disagree with the video then, the pipe has collapsed as air or fluid has escaped when the system was hot, now the system is cold the air/fluid has cooled and shrunk causing the pipe to collapse. The pipe should be normal when cold and under pressure when hot.
    – Solar Mike
    Apr 2 '18 at 12:04

As the the coolant cools off and decreases pressure, a vacuum is created in the cooling system causing your flattened hose. The smaller check valve at the bottom of the radiator cap, (the small metal circle at the very bottom of the cap) should be 'pulled open' by this vacuum, essentially sucking in coolant from the resevoir, into the engine. Unfortunately it is not working properly.


Since you said it was replaced recently, I would have to guess that you have some stop leak in the system, and over time, it has restricted that passage.

I advise you to have your cooling system thoroughly flushed, then replace the cap again.

  • So, if there is a vacuum (just using the term as you did...) in the cooling system then how will the fluid escape into the engine as you say?
    – Solar Mike
    Apr 2 '18 at 12:17
  • I also disagree with the video. The title mentions how to tell if a bad radiator cap is causing coolant overflow by seeing if the top radiator hose is collapsed. A plugged check valve on the cap causes a collapsed hose, not opening or overflowing of coolant under normal pressure. I also agree that there is probably a very small leak when warm allowing a void in the system that isn't getting replaced from the resevoir during cool down due to the plugged check valve in the cap.
    – Milison
    Apr 2 '18 at 12:40

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