Coolant Leak
First I would check water pump or coolant tubes around the TB.
Pressure tester would find the leak quickly.
If you don't see a leak while its running, I'd let it get heat soaked, maybe go for a drive and let it get totally hot. Then come back, put in on the ramps, shut it down and look for leaks for a few minutes. After you shut it down the pressure in the cooling system will rise for a little while, some leaks will only leak at the higher pressures before it starts to cool down.
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If you don't see a leak while its running, I'd let it get heat soaked, maybe go for a drive and let it get totally hot. Then come back, put in on the ramps, shut it down and look for leaks for a few minutes. After you shut it down the pressure in the cooling system will rise for a little while, some leaks will only leak at the higher pressures before it starts to cool down.
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