Coolant in the engine oil would most likely indicate a

• A. Low oil level
• B. Faulty thermostat
• C. Leaking head gasket
• D. Seized water pump

Answer: (C)

Coolant in the engine oil happens when the cooling system and lubrication system start to cross-contaminate, usually due to a head gasket failure. The head gasket seals the cylinder head to the engine block and keeps coolant passages separate from oil passages. If that seal leaks, coolant can seep into the oil galleries, turning the oil milky and thinning its lubrication. This contamination is a hallmark of a leaking head gasket, and addressing it quickly is important to prevent engine damage. A low oil level wouldn’t introduce coolant, a faulty thermostat mainly affects temperature control rather than cross-contamination, and a seized water pump stops coolant flow but doesn’t by itself mix coolant into the oil.