My wife's 85 380 expansion tank had a leak. I replace the tank filled with antifreeze, when I started the car white smoke was coming out the exhaust pipe. She said the temp when up to 100c and she shut the car off. Could the head or head gasket be bad?

If it's leaking into the cylinders that fast (because the system can now pressurize), then you should have exhaust pressure in the coolant tank. Start the car (when cool), and after 30 seconds loosen the exp. tank cap to first detent; if explosive pressure comes out, you have verified a head leak, or at least mandated head removal. This happens because now the exhaust can pressurize the cooling system and push the anti-freeze into the cylinder when on intake (vacuum) stroke. If there is no extreme pressure release, there is a tool to check your anti-freeze for carbon monoxide (exhaust leak into cooling system), available through better tool venders near you.

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