Here's why and what else it could be:
* Blown Head Gasket: This is the most common cause. The head gasket seals the combustion chambers from the cooling system. If it fails, combustion pressure and/or engine oil can leak into the coolant passages. This often results in a milky, brownish sludge in the coolant, as well as oil in the coolant tank.
* Cracked Cylinder Head: A crack in the cylinder head can also allow oil to mix with the coolant. This is less common than a blown head gasket, but equally serious.
* Cracked Engine Block: While less frequent, a crack in the engine block can also lead to oil contamination in the coolant. This is usually accompanied by other symptoms, like external leaks.
* Oil Cooler Leak: Some engines have an oil cooler integrated into the cooling system. A leak in this cooler can allow oil to enter the coolant.
What to do immediately:
1. Stop driving the car immediately. Continuing to drive with oil in the coolant can cause severe engine damage, potentially requiring a costly rebuild or replacement.
2. Do not add more coolant. Adding more coolant without addressing the root cause will just mix more oil and worsen the problem.
3. Have the vehicle professionally diagnosed. A mechanic will need to perform a pressure test on the cooling system and possibly a compression test to determine the exact cause of the oil contamination. They may also need to inspect the cylinder head and block for cracks.
Ignoring this problem will almost certainly lead to a much more expensive repair down the line. The sooner you get it checked, the better the chances of a less severe (and less costly) fix.
