Is coolant not suppose to decrease over time?
Not too long ago (maybe 3~4000miles ago?), the coolant level was near LOW mark. I filled it up to MAX.
Today, i checked the level again and the coolant is at LOW mark again.
I never changed the coolant before. MAYBE the coolant was changed when i bought the car (about 10000 miles ago).
This exact thing happened to my brother's g35. But his coolant was changed about 10000miles ago by me.
now im wondering if i should change the coolant, or keep adding coolant.

 

coolant gets filled at the radiator not at the overflow - that's why it's called an overflow tank

follow the factory service manual for the filling/bleeding procedures

 

Its a closed system. THere is no place for the coolant to go.

 

It's not really closed. There's a small hose from the cooling system to the bottom of the overflow tank. You can see the hose near the radiator cap.

If there's any air in the system, it'll expand when the engine get hot, and that'll force some coolant into the overflow tank. It will be pulled back into the system as it's cooling.

If things are working like they should, there will be very little air in the system. It'll eventually collect at the high spot under the radiator cap, and go into the overflow tank. It'll be replaced with coolant when the engine cools.

As long as the cooling system is working OK and there are no other symptoms (white smoke, obvious leaks, etc), I wouldn't be concerned. Just check it regularly, and keep adding coolant as needed. It really doesn't take much coolant to go from min to max on the reservoir.

 

Other than horrible stinky exhaust smell from my bro's car, everything works like they should be.
This time i'll add coolant through the rad neck and see what happens. thanks