Where is the thermostat on a Chevy Lumina 3100 engine?

The thermostat on a Chevy Lumina with the 3100 engine is located on the engine, at the back of the engine block, near the water pump. It's not easily accessible and usually requires removing some components to get to it. You'll likely need to consult a repair manual specific to your year and model Lumina for exact instructions and diagrams.