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When you
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When you winterized it, did you run the engine for a while to circulate the antifreeze? If not, the block could crack where the antifreeze did not mix. If cracked to the outside, coolant could have leaked out during a thaw then evaporated or run out the drain hole long before you started it. If cracked internally, coolant could run to the oil pan.
Coolant could also get out via the risers and out the exhaust.
Check the dipstick for water.
Fill the heat exchanger and pay attention to how much it takes to fill it. Run the engine to temp and see if the coolant level changes. Add more coolant if needed to determine if coolant level will stablise.
You can smell hot antifreeze if it is leaking via a bad hose connection. You can see a steam cloud with antifreeze smell if leaking via cracked head. If leaking via a internal leak at the risers, you will might see green in the exhaust water"