Here's why and what to check:
* Head Gasket Failure: This is the most common cause. A compromised head gasket allows coolant to seep into the engine's cylinders or oil passages. This will often lead to milky oil (looks like a milkshake) due to the coolant mixing with the oil. You should check your dipstick for this.
* Cracked Cylinder Head: A crack in the cylinder head can also allow coolant to enter the engine's internal passages. This is less common than a head gasket failure but can be just as serious.
* Cracked Engine Block (less likely): While less common, especially without other symptoms, a crack in the engine block itself is possible. This would also result in coolant entering the oil.
* Very slow external leak: While you state there's no visible leak, consider extremely slow leaks that might evaporate before you can see them. Carefully inspect all hoses, clamps, radiator, and water pump for even minor dampness or residue. Pressure test the cooling system for even small leaks.
What to do:
1. Check your oil: The most crucial step. Is your oil milky or frothy? This strongly indicates coolant is mixing with the oil.
2. Pressure test the cooling system: A mechanic can pressure test the cooling system to pinpoint a leak, even tiny ones. This will show if coolant is escaping under pressure.
3. Inspect the entire cooling system: Carefully examine all hoses, clamps, radiator, water pump, thermostat housing, and heater core for any signs of leaks, no matter how small.
4. Check the coolant overflow tank: Make sure the level is dropping consistently.
5. Consider a combustion gas test: This test checks for combustion gases in the coolant. Positive results strongly suggest a head gasket leak.
If you find milky oil or a pressure test reveals a leak but you can't visually find it, it's highly probable that you have internal engine damage requiring professional repair – likely a head gasket replacement or more extensive work. Ignoring this could lead to catastrophic engine failure. It's best to have a mechanic diagnose the problem promptly.
