* Faulty Heater Core: A heater core leak allows coolant to leak into the passenger compartment, but it also means coolant is escaping the main cooling system. If the leak is significant enough, it can lead to low coolant levels and potentially contaminate engine oil if the leak is internal within the engine block (though usually the leak manifests externally first).
* Faulty Water Pump: A failing water pump seal can allow coolant to leak into the engine oil. The water pump is directly connected to the engine's cooling system, and a breach in its seal allows mixing of coolant and oil. This is a serious problem requiring immediate attention.
In both cases, the presence of coolant in the engine oil is indicated by a milky or chocolate brown appearance of the oil, and often a sweet smell. This is a critical issue that needs immediate repair to prevent engine damage.
