* Cooling System Issues: This is a prime suspect. The light might be triggered by:
* Overheating: A failing thermostat, water pump, radiator fan, low coolant level, or a leak in the cooling system could cause the engine to overheat after it's been running for a while under load. The longer you drive on the highway, the hotter the engine gets, until the sensor triggers the light.
* Low Coolant Level Sensor: A faulty sensor might not register the low coolant level until the engine reaches operating temperature.
* Oxygen Sensor (O2 Sensor): These sensors work best when hot. A failing O2 sensor might not function correctly until the engine reaches operating temperature, triggering the light after the warm-up period.
* Mass Airflow Sensor (MAF Sensor): Similar to the O2 sensor, the MAF sensor's readings might become inaccurate when hot, leading to a lean or rich air/fuel mixture which can trigger the check engine light.
* Ignition System Problems: Worn spark plug wires, distributor cap, or rotor could cause misfires that become more noticeable at higher engine speeds and temperatures.
* Transmission Issues (Less Likely): Though less probable, issues with the transmission fluid temperature or a sensor related to the automatic transmission could trigger the light.
* Exhaust System Leak: A leak after the catalytic converter can also throw a code, and might get worse as the engine and exhaust system heat up.
Troubleshooting Steps:
1. Check the coolant level: This is the easiest and most important first step. Low coolant is a serious problem.
2. Obtain a diagnostic trouble code (DTC): You'll need an OBDI scanner (designed for older vehicles like your 1991 Corsica) to read the code. Auto parts stores often offer free code reading services. The code will give you a much more specific clue about the problem.
3. Inspect visually: Look for obvious signs of leaks in the cooling system, hoses, etc. Check the condition of spark plug wires, the distributor cap and rotor.
4. Check the radiator fan: Make sure it's turning on when the engine reaches operating temperature.
Important Note: Driving with a check engine light on can cause further damage to your engine. It's crucial to address the problem as soon as possible. Don't ignore it. The heat-related possibilities, especially overheating, are particularly serious and can cause catastrophic engine failure.
