Funky engine coolant issue
 

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.

Engine coolant issue
 

Do a compression test!! see if the cylinders holds compression.

These are one of the best way to see if your head gasket or even the cylinder head is cracked.

White smoke can be caused by running rich on fuel also.

oil in radiator or overflow, water in oil does not need to be the only indicator of these issues.

Hope these works