Other, less likely possibilities include:
* Cracked cylinder head: A crack in the cylinder head itself can also allow oil and coolant to mix. This is often accompanied by other symptoms like overheating.
* Cracked engine block: Less common than a head gasket failure, but a crack in the engine block can also lead to the same mixing.
* Coolant passage leak: A leak in a coolant passage itself, perhaps due to corrosion, could allow oil to enter. This is less likely to cause oil to appear in the coolant reservoir.
Why the head gasket is the most likely culprit: The 3.1L diesel engine in the Grand Cherokee, being an older engine, is more prone to head gasket failure due to factors like age, wear, and potential overheating issues. The pressure from combustion is the most likely cause for oil being forced into the cooling system.
Important Note: Driving with oil in your coolant is extremely dangerous. The oil can clog the cooling system, leading to overheating and catastrophic engine damage. Do not drive the vehicle until the problem is diagnosed and repaired. Have it towed to a qualified mechanic for proper diagnosis and repair.
