I believe this vehicle has an expansion tank (as opposed to the traditional overflow tank design with check valve in the rad cap), so what you are seeing in all likelihood is air bleeding out of the system slowly over time since you were the one who originally discovered that the coolant was low. It may have been low from someone doing a flush or coolant exhange improperly and not bleeding the system properly. Continue to monitor for coolant buildup in the water pump/serpentine belt drive area, on the radiator and coolant hoses, and of course watch for white smoke coming out the tailpipe (head gasket).

That is indeed very odd, hard to say what is going on, this could be why the seller sold you this car . Do check the exhaust for while smoke of a sweet coolant smell, but I fear your head gasket is leaking thus your burning a little coolant. I would ask your local Hyundai dealership if they have ever heard of this .

You keep talking about the reservoir, but did you ever check the radiator? They both have to be full. If your radiator wasn't topped off, it would make sense that the reservoir level would fall, even if the engine wasn't too hot.

check under the carpet on the passenger side you could have a leaking heater matrix, water will evaporate very quickly you must have 50/50 coolant

Hi,

Recently I bought a 07 Elantra with 100K miles on it. Since I bought the car, I noticed that the cap of the coolant reservoir tank was not properly closed. It was loosely placed on top of the reservoir mouth. Last night, just for curiosity while I was checking the coolant level, I found that it was really low (way below the minimum mark on the reservoir). I then filled the reservoir and marked the level. After that I drove that car for around 10 min. 2 hrs. Later I again checked the coolant level and found that the level dropped by about half of an inch. I also, placed a white sheet under my car to look for any visible leakage. This morning before driving the car I again checked the coolant, and found that it dropped again by another half of an inch. However, I did not see any leakage on the white sheet. I took the car to a mechanic shop and asked them to do a coolant flush. Also, asked them to do a pressure test. They said that the pressure testing was fine. By the way I never had any engine heating issues. My queries are:

1. Does the coolant get evaporated if the reservoir cap is loose while driving?

2. Is it normal that the coolant level goes down that much when the engine is cold?

3. What else could I do by myself to make sure that there is no leakage and everything is working fine?

Thanks