* Cracked or damaged engine block: This is the most serious possibility. If the block itself is cracked near a coolant passage and an oil passage, you'll get a mixture of both fluids leaking. This usually requires a major engine repair or replacement.
* Cracked or damaged cylinder head: Similar to a cracked block, a crack in the cylinder head can allow coolant and oil to mix and leak. This is also a significant repair.
* Head gasket failure: This is a very common cause of coolant and oil mixing. The head gasket seals the combustion chambers from the coolant passages and the oil passages. A blown head gasket allows them to intermix, leading to leakage. This is a costly but relatively common repair.
* Oil cooler leak (if equipped): Some engines have an oil cooler that's integrated into the cooling system. A leak in this cooler could mix the fluids and cause leakage.
* Damaged freeze plug: While less likely to mix oil and coolant directly, a damaged freeze plug (a type of plug that seals coolant passages) *could* be close enough to an oil passage that it causes a leak that appears to be both.
Determining the exact cause:
It's impossible to diagnose the specific problem without a proper inspection. You'll need to:
1. Visually inspect the engine: Look carefully for the exact location of the leak. Take photos if possible. Pay attention to any staining or discoloration that might indicate the source.
2. Pressure test the cooling system: This will help identify leaks in the cooling system components.
3. Check the oil for signs of coolant contamination: Milky or foamy oil indicates coolant in the oil.
4. Have a mechanic perform a compression test: This will test the integrity of the cylinder head and the head gasket.
5. Leak detection dye: A mechanic might introduce a dye into the cooling system to help pinpoint the leak.
Important Note: Driving a car with a coolant and oil leak is extremely dangerous. Running the engine low on oil will cause catastrophic engine damage. Overheating from coolant loss is also damaging. Have this problem investigated and repaired immediately by a qualified mechanic.
