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Large Coolant Leak - Don't know where it's going

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I have a 2007 997 4S Cabrio w/48k miles that started leaking coolant on a large scale. I lose about 1 gallon (4L) of coolant now, but there are no visible signs on the floor of my garage.  It seems to need to be up to temperature for the leak to occur. After it occurs, I lose a lot of coolant, and the coolant low warning light can come on. The first time it happened, there was a lot of steam in the engine compartment. But I haven't seen that since the first time. 


Sometimes when this happens, I smell the coolant in the car, sometimes not. But can't find the coolant anywhere. I've now replaced the water pump and the coolant expansion tank. The oil is not chocolate milk - it looks normal. But we are getting it tested for glycol. Initial test was negative, but the more extensive test results are coming today. One clue is the oil level seems to have increased just over the top line of the electric indicator (but not blinking). Maybe trace amounts getting into the oil? Where's the rest going? There is no sweet smell coming from the exhaust.


I put some UV tracer in the coolant and have now driven it to lose coolant. However, I can't see any large traces of the UV dye anywhere. The area around the overflow cap doesn't have any significant residue - just seeming splatters (i've ordered a new cap). I'd like them to check it out today with the UV light and take off all the under panels - maybe the radiator? But wouldn't that show up on the garage floor? 


Indie shop is more and more convinced it's a crack in the cylinder head or some other part of the engine like the block - and the majority is venting to the outside - but wouldn't I see smoke? 


Has anyone seen anything like this? 

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