Here's why and what's involved in fixing it:
* Head Gasket Failure: The most likely culprit is a blown head gasket. The gasket seals the combustion chamber from the oil gallery. If it's damaged, oil can leak into the cylinder.
* Cracked Cylinder Head: Less likely, but possible, is a crack in the cylinder head itself. This allows oil to seep into the combustion chamber.
* Other possibilities (less likely given the location): While less likely given the oil is specifically in the #4 spark plug well, worn valve stem seals could contribute. However, a head gasket failure is far more probable with oil directly in a spark plug hole.
What needs to be done:
A proper repair involves a significant engine overhaul:
1. Diagnosis: A mechanic needs to perform a compression test and a leak-down test to confirm the extent of the damage and pinpoint the exact source of the leak.
2. Head Removal: The cylinder head will need to be removed. This involves removing various components like the valve cover, intake manifold, etc.
3. Head Gasket Replacement: The head gasket will need to be replaced. Often, the head bolts are also replaced.
4. Cylinder Head Inspection: The cylinder head will be inspected for cracks or warping. If it's damaged, it may need to be resurfaced or replaced.
5. Block Inspection: The engine block will also be inspected for damage.
6. Reassembly: After all repairs and inspections, the engine will be reassembled.
7. Testing: After reassembly, the engine will need to be tested to ensure the problem is resolved.
In short: This is not a DIY job for most people. It requires specialized tools, knowledge, and experience. The cost of repair will depend on the extent of the damage and whether the cylinder head needs to be replaced or just resurfaced. Take your S10 to a qualified mechanic for diagnosis and repair. Ignoring this problem will likely lead to catastrophic engine failure.
