If there is no leak? So how does the coolant level decreases as time passes? Does it get pressurized so it's level decreases? Or what happens? Since it keeps circulating to the engine and coming back so why do we have to add some coolant every year or something, even if it evaporates inside it will again condensate after the engine is off and cooled

  • possible duplicate of Reasons for loss of radiator fluid
    – Zaid
    Sep 26, 2015 at 15:56
  • @Zaid - I think there is some overlap here, but the gist of this question is different than is your question ... I'm not willing to vote to close at this point, but definitely see your point. Sep 26, 2015 at 16:00
  • good question!!
    – Shobin P
    Sep 26, 2015 at 18:40
  • Water evaporates it's nearly impossible it will drop overtime . This is why after service or coolant change keep remainder in trunk to top off. And I would only measure overflow tank as vehicle reaches operational temperature. Cold level is bogus especially if there is a air pocket in system Apr 20, 2020 at 18:37

1 Answer 1


There are only four places (in general) coolant can go missing:

  • Out an external leak and onto the ground
  • Out through the tail pipe getting burnt (turned into steam) because of an internal leak
  • Into the oil through an internal leak
  • A very small amount may evaporate from the system if it's not a closed system (ie: no overflow reservoir) ... this really isn't a big deal, just add a small amount to make up the difference. (note: this fourth one was added as an edit)

Since coolant is a liquid, it does not compress, even if pressurized.

The first is pretty obvious as you can find it on the ground, usually. A leak may not occur until the system is fully pressurized, though (ie: after the entire system is fully heated). You would usually see this going on due to steam escaping the system.

The second is not as easy to tell, especially if it's a small leak. You could have a very small leak in the head gasket, which would go out through the exhaust. You would probably not see this going on, as when gasoline is burned, it creates a large amount of steam as well. You may be able to smell a sweet smell from the exhaust.

The third way you may or may not see as well, depending on the amount of coolant loss. If it is just a very small amount, it will mix in with the oil and get evaporated. If it's a larger amount, it will turn the oil to look like chocolate milk.

If you have just a small amount of coolant missing over time, a radiator stop leak (if used correctly) will find and stop the leak without issue.

  • So it shouldn't be decreasing at all? Even not adding some coolant every year, because that what i was taught, if it decreases a very small amount over time, is there a leak as well? Sep 26, 2015 at 16:12
  • I think what they are telling is is it's not a big deal if you lose a little bit over time. You will lose a very small amount due to evaporation unless the entire system is sealed (no overflow tank). My opinion is, unless it's an obvious amount (ie: like having to add a quart per week - and that's an arbitrary number on my part), it's really not a big deal. Sep 26, 2015 at 16:24
  • Yeah I know it is not a big deal I am just wondering where it goes Sep 26, 2015 at 16:29
  • I guess I should add in a "fourth" way you can lose coolant in my answer, eh? :D Sep 26, 2015 at 16:31
  • If you have a system with an overflow tank, it is still "sealed" unless the whole cooling system is unpressurized. But when you top up the system, it's quite likely the pipe between the main system and the expansion tank gets full or air not coolant, so the first time you run the engine the coolant level apparently goes down a bit as that air ends up in the expansion tank and the pipe stays full of liquid. The only times I've had a significant amount of coolant loss, in 30+ years motoring, has been caused by a leak somewhere. Usually the filler cap doesn't get opened for years on end.
    – alephzero
    Sep 26, 2015 at 18:17

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