If engine oil is found in the coolant, which component is leaking?

• A. Injector cups.
• B. Oil cooler core.
• C. After cooler core.
• D. Cylinder liner O-rings.

Answer: (B)

Oil in the coolant points to a failure in the oil cooler core. The oil cooler is a dedicated heat exchanger that runs engine oil on one side and coolant on the other, with a separating wall and seals between them. If the core develops a crack or a failed seal, oil can leak into the coolant loop, contaminating it. That direct path between oil and coolant is unique to the oil cooler core, which is why it’s the best answer. The after cooler core isn’t the usual source of oil entering coolant, and injector cups or cylinder liner O-rings would typically cause different symptoms (such as oil leaks elsewhere or coolant/oil mixing due to other seal failures) rather than the specific cross-contamination described here.