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
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.