When I have a small coolant leak or coolant loss on a car like this Honda Civic, the first things I always check is.. look for obvious leaks on hoses and the radiator tanks, then I inspect the radiator cap. I am looking for rotted, cracked, dry seals or a weak spring on the radiator cap. If the car has the original radiator cap.
The radiator cap is responsible for keeping the cooling system under a specific pressure, if the seals are faulty or the spring is worn, the cooling system will not hold pressure and the radiator cap will allow coolant to escape, and then once enough coolant is lost, the car will start overheating.
Radiator caps usually cost between $10-$15 for most cars, so if you suspect it, might as well replace it to eliminate it as a suspect in your coolant loss issue.
If you are losing coolant but there is no visible leak, check your radiator cap!
