My coolant tank is filled up to the correct level before driving off but I've taken a half an hour drive and it is very low and warning light comes on. Once stopped it cools down and the level rises. I've had it checked and theres no leak. Any ideas why it goes low very quickly and how I can sort this out?

Thanks Anthony

  • 1
    By coolant expansion tank, do you mean something like what is pictured in this question ? This image might also be helpful.
    – JPhi1618
    Commented Apr 13, 2016 at 19:22
  • So is there really a leak if the level goes back to normal once the engine is cool?
    – JPhi1618
    Commented Apr 13, 2016 at 19:27
  • What warning light is coming on? Is it the temperature warning light or do you have a light for low coolant? Commented Apr 13, 2016 at 19:36
  • 2
    This sounds like a very strange behavior! The only thing I can imagine would be that you have a rather large air pocket in your system, and as the water gets hot, it compress the air, and once it gets cold the air force the water back. It's a long shot, but you might want to make sure that the system is properly bleed.
    – Markus
    Commented Apr 14, 2016 at 11:13
  • 1
    might be the air pocket moved out and water got sucked in.
    – Alexus
    Commented Apr 25, 2016 at 22:49

1 Answer 1


Sounds like an air pocket in the cooling system. Take the cap off of the overflow tank and radiator if possible and start the engine and turn the heat on so it'll circulate fully, allow the engine to reach operating temperature and the coolant level should drop as soon as its flushed the air pocket out, top off the system, replace caps and you're good to go.

  • If this doesn't work, consider draining and flushing the cooling system. There is a bleed hole on a heater core hose used specifically to force air out when the cooling system is being refilled. You will need to check the specific procedure for your model. Commented Aug 18, 2016 at 4:32

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