* Blown Head Gasket: This is the most frequent cause. A failing head gasket allows coolant to leak into the engine's cylinders, and subsequently into the crankcase via combustion blow-by. This often results in milky, chocolate-brown oil. You might also see white smoke from the exhaust, overheating, and low coolant levels.
* Cracked Cylinder Head or Engine Block: A crack in either the cylinder head or engine block can also allow coolant to mix with the oil. Similar symptoms to a blown head gasket will be present, but the damage is more significant and usually requires a more extensive repair.
* Faulty PCV Valve (Positive Crankcase Ventilation): While less likely to cause significant water contamination, a faulty PCV valve can allow moisture and other contaminants to enter the crankcase. This typically results in less severe oil contamination.
* Condensation: In colder climates, condensation can form within the crankcase. This is usually a smaller amount and less problematic than the other causes. It's more likely to be noticed after short trips.
* Coolant Leak into Engine: A leak from the coolant system, such as a cracked hose or a leaking radiator, might drip onto engine parts and eventually find its way into the oil pan via drips or spray. This might also lead to low coolant, but the oil might not be as visibly milky.
How to diagnose:
Determining the specific cause requires a thorough inspection:
* Visual Inspection: Check the oil dipstick for milky or creamy oil. Inspect the coolant reservoir for signs of oil contamination (oil sheen). Look for external coolant leaks around the engine.
* Compression Test: This test checks the compression in each cylinder. Low compression in one or more cylinders could indicate a blown head gasket or cracked head/block.
* Leak-Down Test: This test can pinpoint the source of a leak, identifying if the leak is past the head gasket or piston rings.
* Coolant Pressure Test: This test checks for pressure leaks in the cooling system.
* Oil Analysis: A professional oil analysis can reveal the presence of coolant or other contaminants in the oil.
Important Note: Driving with water in your oil can cause serious engine damage. It can lead to lubrication failure, resulting in seized bearings, scored cylinders, and ultimately, engine failure. Have the problem diagnosed and repaired promptly by a qualified mechanic.
