Sounds like you need a Head gasket Diagnosis. Have your mechanic
check for coolant loss into your Combustion chamber. This can be done by using a gas analyzer or a fluid that reacts to combustion
gases in the cooling system. Or pull the plugs and ck for coolant
after it sits and cools off overnight. Does it miss fire briefly when started
cold? If so find the misfiring cylinder and that's where your coolant leak
exists.