It's leaking coolant, probably along the top seam between the plastic top-tank and the aluminum radiator core. It is often difficult to locate this kind of leak, but here are two suggestions:
1) to confirm that it's really a coolant leak, carefully mark the overflow bottle level, say, when the engine is cold (never started for the day). Then, keep watching to see if the level returns to that mark the following day(s). Also, watch this level when the engine is hot - it's going up, right? A properly sealing radiator / cooling system will self-purge any air, at which point the overflow bottle level will only reflect system temperature.
2) the time of greatest system pressure is when you come off the expressway after a long drive and immediately shut the engine down. Both the cooling system temperature and pressure rise significantly. So, next time you pull off the highway, pull over to the roadside or a parking area and pop the hood. Look carefully for steam or 'smoke' or other things coming from the top area of the radiator.
FWIW, your radiator has a lot of excess capacity. So, just because it's not holding full pressure (i.e. leaking) doesn't mean your engine will overheat. It probably won't overheat unless you've got an internal leak - combustion gasses escaping the cylinder(s) and entering the cooling system.
1) to confirm that it's really a coolant leak, carefully mark the overflow bottle level, say, when the engine is cold (never started for the day). Then, keep watching to see if the level returns to that mark the following day(s). Also, watch this level when the engine is hot - it's going up, right? A properly sealing radiator / cooling system will self-purge any air, at which point the overflow bottle level will only reflect system temperature.
2) the time of greatest system pressure is when you come off the expressway after a long drive and immediately shut the engine down. Both the cooling system temperature and pressure rise significantly. So, next time you pull off the highway, pull over to the roadside or a parking area and pop the hood. Look carefully for steam or 'smoke' or other things coming from the top area of the radiator.
FWIW, your radiator has a lot of excess capacity. So, just because it's not holding full pressure (i.e. leaking) doesn't mean your engine will overheat. It probably won't overheat unless you've got an internal leak - combustion gasses escaping the cylinder(s) and entering the cooling system.
