What is the mileage of your vehicle? Does it leak coolant on the floor when parked? if the answer is no, consider the possibility of a leaking head gasket. a simple way to check would be opening the radiator cap with the car running, but not hot or at normal temp,but not cold either. basically you want no pressure in the cooling system during this check. if you get bubbles only with the car running,it means the head gasket is probably shot and is leaking compression into the cooling system. or if you have coolant in the oil, then the breach is from high pressure coolant passage to an oil return that is normally separated by the head gasket but is now been breached. or just try a cooling system pressure test with the engine off and the plugs out to see if coolant leaks into the combustion chamber or even if you hear hissing from the oil cap opening.