always low on coolant

How long does it take for the coolant to leak down and are you certain you are filling it all the way? I would have a shop pressure test the cooling system to see if they can pinpoint the leak. The coolant should not be low if there is no leak. Some leaks are slow enough that they'll evaporate before reaching the ground and you can look for the trails or evidence where coolant may have leaked externally. Otherwise, your head gaskets may be starting to leak coolant into the cylinders. Check it thoroughly so you don't unnecessarily waste money on parts. hope this helps!

Andy is certainly correct. I would get a repair shop to pressurize the cooling system to check for leaks but I would then have them do what is called a "block check". It is a chemical check to look for the presence of carbon monoxide in the cooling system which would only be present if the head gasket is not sealing. The 2.5 liter engines do experience a lot of head gasket failures.