Help! Heater Box overheating?

[raycow]

Definitely check for leaks first. Now after you don't find any......
When you shut down an engine, water in contact with hot spots can start to boil locally, since there is no longer any circulation to carry away the heat. This is not harmful in itself. However, the vapor bubbles created by the boiling are lighter than water and want to migrate to high points in the cooling system, which just happen to include the heater core.

Ray