I had one come in with bad head gaskets and if you took the cap off at idle when cold.(or hot maybe didn't check it that way) and watched the coolant level in the tank after about 30 seconds it would start to raise the lever for 20-30 seconds and then after if came up a couple inches it would burb a whole bunch of gases and then go back down. then it would start all over raising and then burb. guy was using coolant but not seeing it anywhere. both gaskets were bad and had been for along time.