1. Valve Cover Gasket Leak: The valve cover gasket is a seal between the valve cover and the cylinder head. If this gasket fails or becomes damaged, oil can leak from the valve cover and drip down into the spark plug wells.
2. Spark Plug Tube Seal Leak: Each spark plug has a rubber seal that helps prevent oil from leaking into the spark plug well. Over time, these seals can deteriorate and allow oil to seep into the wells.
3. Piston Ring Wear: Excessive wear on the piston rings can cause oil to leak past them and into the combustion chamber. This oil can then travel down the cylinders and accumulate in the spark plug wells.
4. Faulty PCV Valve: The positive crankcase ventilation (PCV) valve regulates the flow of gases from the crankcase to the intake manifold. If the PCV valve fails or becomes clogged, it can cause pressure to build up in the crankcase and force oil into the spark plug wells.
To determine the exact cause of the oil in the spark plug wells of your 1990 Corolla, it's recommended to inspect the valve cover gasket, spark plug tube seals, piston rings, and PCV valve. Replacing any faulty components should resolve the issue.
