coolant leak

You can add a small bottle of dye to the coolant. After driving, you search with a black light including at the tail pipe. The dye will show up as a bright yellow stain. This is particularly useful when there is no puddle or other sign of external leakage. You can also use a pressure tester at the radiator cap to pressurize the system and search around hoses and engine gaskets. If you see any drips between the two front tires right in front of the firewall, suspect a leaking heater core.