Funky engine coolant issue

[shirkinator]

Hey everybody I have a 1988 Supra 7mgte. I bought it last week and when I bought it the heater did not work. The fan blows but only cold air. The temp guage stays normal. Until last night. The engine started running rough. The temp went all the way up, and Im blowing white smoke out the exhaust. I was low on coolant the next day. I have no oil in my coolant and no coolant in my oil and I had my coolant tested and it had no trace of exhaust in it. So I know that my head gasket is not blown. What would be the next logical problem. Would it be a blown intake manifold gasket, or coolant coming in through the turbo, or any other ideas. I have no idea here. Any knowledgeable help is greatly appreciated. Thank you very much.