What causes coolant to " evaporate " from overflow tank?

Hello, first, coolant doesn't evaporate. Nor is it consumed by the engine - during normal operation. If the coolant level goes down it's because there's either been an air pocket in the cooling system that has finally purged out, or there's a leak of coolant, either internally in the engine or externally outside of the vehicle. Add Coolant if needed to the proper level and monitor. You will likely find that the coolant level is accurate and not a cause for concern. https://repairpal.com/auto-repair-near-me Click the link or call us at 877-913-1554. We'll gladly find you a certified RepairPal shop in your area.