Engine oil contaminated with coolant is most likely due to which failure?

• A. Cracked dry liner.
• B. Leaking liner O-rings.
• C. Leaking oil cooler core.
• D. Cracked liner counter-bore.

Answer: (B)

coolant in the oil means there’s a breach between the coolant side and the oil side. In engines with wet liners, the line between those fluids is sealed by O-rings around the liner. If those O-rings fail, coolant under pressure from the cooling loop can seep into the oil galleries and contaminate the crankcase oil, often turning it milky. That direct seal problem is the most common and straightforward route for coolant to mix with oil, which is why leaking liner O-rings is the best answer. Other failures like a cracked dry liner or an oil cooler core leak can also cause contamination, but they are less likely to produce the same direct and noticeable oil–coolant crossover path as a failed liner O-ring.