It must be leaking internally. Have you had a cooling pressure test done yet? That would be the first step, then see if the pressure drops. If the pressure doesn't drop, you may have a very slight leak based on temperature. You could perform an engine leakdown test and check for air bubbles in the radiator/coolant to see if the headgasket is leaking.
Saturday, March 7th, 2015 AT 8:19 AM