Here's why this is happening and what you should do:
* Failed Head Gasket: This is the most common culprit. The head gasket seals the combustion chambers from the coolant passages and the oil galleries. A failure allows coolant to leak into the oil and vice-versa. This often manifests as milky, brownish sludge in the oil or, as you've found, oil in the coolant.
* Cracked Engine Block or Cylinder Head: A crack in either the block or the cylinder head can also allow coolant and oil to mix. This is a more significant problem than a failed head gasket, usually requiring more extensive (and expensive) repairs.
* Improperly Installed Head Gasket (less likely): If the head gasket was recently replaced, it's possible it wasn't installed correctly. This is less probable than the other causes, but it's still worth considering if you've had recent engine work.
What you should do:
1. Stop driving the car immediately. Continuing to drive with oil in the coolant will cause further damage to the engine. The mixture can clog cooling passages and lead to overheating, potentially causing catastrophic engine failure.
2. Have a mechanic diagnose the problem. They can perform a pressure test on the cooling system and inspect the oil for contamination. A visual inspection of the engine might reveal external signs of leakage. A compression test can also help pinpoint the location of the problem.
3. Prepare for repairs. The cost of repairs can vary greatly depending on the extent of the damage. A head gasket replacement is a major repair, while a cracked block or head might require a replacement engine or extensive machine work. Get multiple quotes from reputable mechanics before proceeding.
Do not attempt to fix this yourself unless you have significant mechanical experience. This is a serious problem requiring professional attention.
