Honda civic 2011 103k kms. I have flushed my radiator around 6 months ago. Since then my coolant reservoir level fluctuates. I havent used the car for 3 days then I noticed that the coolant goes below 1 inch from minimun but when the car warms up it goes at half mark. I kept on adding coolant on the reservoir but after few wweks coolant goes below Minimum line when cold but it never drains completely to nothing. Just An inch below min when Cold in the morning
Of course the level will change with temperature, but if you are actually losing coolant it is either leaking or burning. I would pressure test the cooling system and/or add some UV dye to see if there's a small leak somewhere.
Water expands and contracts like air does when you change the temperature (not nearly as much, but it does). That's why there's an expansion tank. If the expansion, also called the overflow tank weren't there, the cooling system would need to be designed to withstand much larger forces than it does.
If you didn't get all of the air out when you refilled your cooling system, the system may eventually self-bleed if you top up a few times. If the problem didn't exist before you did the coolant change, this is likely your issue.
In addition to checking for leaks or burning coolant by pressure testing, as @chucktobias says, check your that your radiator cap is seated properly and consider replacing it. The radiator cap maintains system pressure and vents if it gets too high. The spring tends to wear out over time. The system maintains higher than atmospheric pressure to raise the boil point of the coolant. When the spring wears out, it lowers the pressure, which lowers the coolant boiling point, letting coolant slowly leak as steam.
