Here's why:
* Head Gasket's Role: The head gasket seals the combustion chambers from the cooling system. It prevents coolant from leaking into the cylinders and oil from leaking into the cooling system.
* Failure Mechanism: A damaged head gasket allows a passageway between the engine's cooling system (radiator) and the engine's lubrication system (crankcase). Pressure from either system can force fluid across this breach.
* Other (Less Likely) Possibilities: While a blown head gasket is the most common culprit, other possibilities exist, though they're significantly less probable:
* Cracked Engine Block or Cylinder Head: A crack in the block or head itself could also allow communication between the oil and coolant passages. This is less common than a head gasket failure, but a serious problem.
* Faulty Oil Cooler: Some engines have an oil cooler integrated into the cooling system. A leak in this cooler could allow oil to enter the coolant. However, this usually results in oil in the coolant but not necessarily coolant in the oil.
Diagnosis:
To confirm a blown head gasket, a mechanic would likely perform several tests, including:
* Coolant Pressure Test: Checking for pressure loss in the cooling system.
* Compression Test: Checking the compression in each cylinder to identify potential leaks.
* Leak-Down Test: A more precise test to pinpoint the source of leaks in the cylinders.
* Visual Inspection: Examining the head gasket itself after removing the cylinder head (if necessary). They might look for signs of oil residue in the coolant passages.
* Oil analysis: checking for coolant contaminants in the oil.
In short: Don't drive the car until this is diagnosed and repaired. Running the engine with oil in the coolant can cause serious damage. The milky appearance of the oil is a key indicator. Get it to a mechanic ASAP.
