* Cracked engine block or head: This is a serious problem. A crack allows coolant to leak into the engine's oil passages. This is often accompanied by white smoke from the exhaust, overheating, and milky-looking oil.
* Blown head gasket: The head gasket seals the combustion chambers from the coolant passages. A blown head gasket allows coolant to mix with oil, or leak into the cylinders, leading to similar symptoms as a cracked block/head.
* Leaking coolant passages: Smaller leaks in the coolant passages within the engine block itself can seep into the oil over time.
* Faulty thermostat: A stuck-closed thermostat can cause excessive pressure in the cooling system, leading to leaks. While it might not directly cause water *in* the engine block, it can contribute to leaks that do.
* Improperly installed engine components: During engine rebuilds or repairs, if parts aren't installed correctly, leaks can occur.
* Frozen engine: In extremely cold climates, freezing coolant can crack the engine block or head.
The presence of water in the engine block is serious and usually requires professional attention. Continuing to run an engine with water in the oil can cause significant damage, leading to costly repairs or even engine failure.
