Cracked Cylinder Head or Engine Block.

Internal coolant leaks can also occur if the cylinder head or engine block has a crack in a cooling jacket. A combustion chamber leak in the cylinder head or block will leak coolant into the cylinder(s). This dilutes the oil on the cylinder walls and can damage the piston and rings. If the coolant contains silicates (green antifreeze), it can also foul the oxygen sensor and catalytic converter. If enough coolant leaks into the cylinder, as when the car sits overnight, it may even hydro-lock the engine and prevent it from turning over when you try to start it.