What component is likely leaking if engine oil is discovered in the coolant?

When engine oil is discovered in the coolant, it is most commonly indicative of a failure in the oil cooler core. The oil cooler is responsible for maintaining proper operating temperatures for engine oil and is typically located somewhere in the coolant circuit. If this component develops a breach, it can result in direct contamination, allowing oil to mix with the coolant. This is a serious issue, as the mingling of these two fluids can lead to significant engine performance problems, overheating, and, in severe cases, engine damage.

The other components mentioned, such as injector cups, after cooler cores, and cylinder liner O-rings, typically have different functions or are linked to different types of fluid leaks. Injector cups primarily deal with fuel system integrity, after cooler cores relate to charge air cooling rather than oil, and cylinder liner O-rings are more associated with coolant and combustion chamber seal integrity, affecting engine compression rather than oil flow. Thus, the presence of engine oil in coolant strongly suggests a failure in the oil cooler core rather than issues related to these other components.