* Sensor Disruption: The repair process might have inadvertently damaged or disturbed a sensor near the manifold. Oil leaks often occur near sensors, and the repair might have moved wires, connectors, or even broken a sensor (like an oxygen sensor, mass airflow sensor, or even a crankshaft position sensor). The oil leak itself may have even caused subtle damage to a sensor that only manifested after the leak was repaired.
* Vacuum Leak: The repair might have introduced a vacuum leak. Manifold gaskets are crucial for maintaining proper engine vacuum. If the repair wasn't perfectly executed, a small leak could affect the engine's operation and trigger the SES light. This is especially likely on older vehicles like the 1989 Eldorado.
* Oil on Sensors: Even after cleaning, residual oil could remain on sensors, causing them to malfunction temporarily or until the oil fully evaporates or is cleaned properly.
* Low Oil Level: If the oil leak was significant, even after the repair, the oil level might still be low. This can trigger the SES light (often indirectly; low oil pressure triggers various sensors that in turn set the light).
* Computer Reset Needed: Sometimes, simply disconnecting the battery for a few minutes (to reset the onboard computer) can clear a temporary SES light trigger. This should only be attempted *after* investigating the other potential causes first.
* Unrelated Issue: The oil leak repair might have been completely unrelated to the cause of the SES light. The light could have been triggered by a pre-existing issue that was coincidental with the repair.
What to do:
1. Check the oil level: Ensure the oil level is correct.
2. Inspect the area around the repair: Look carefully for any damaged or loose wires, connectors, or sensors near the manifold.
3. Check for vacuum leaks: A visual inspection might reveal loose vacuum hoses or damaged gaskets. A shop might use a smoke test to find small vacuum leaks.
4. Use an OBD-I scanner: A scanner specific to OBD-I (the standard for 1989 vehicles) will retrieve the diagnostic trouble codes (DTCs) stored by the car's computer. This will give a much more precise indication of the problem.
5. Clear the codes (after addressing any found problems): Only attempt to clear codes after addressing any issues discovered and ensuring that the root cause has been addressed. Otherwise, the light will likely return.
Without using a scanner to retrieve the DTCs, diagnosing the exact cause will be difficult. A mechanic specializing in older Cadillacs would likely have the necessary tools and experience to properly diagnose and resolve the issue.
