* Low Coolant Level: This is the simplest and most important thing to check first. Low coolant means the engine can overheat easily, especially under load. The thermostat might be opening too soon, letting the coolant get too cold while driving.
* Thermostat Stuck Open: The thermostat regulates coolant flow. If it's stuck open, the engine won't reach optimal operating temperature, especially under the increased cooling effect of driving at highway speeds. At idle, there's less demand on the cooling system, so it might still get warm enough.
* Water Pump: A failing water pump isn't circulating coolant effectively. This is more likely to show up under load (highway driving) when the engine generates more heat. You might hear unusual noises from the water pump if it's failing.
* Radiator Issues: A clogged radiator, a failing radiator cap (allowing pressure loss), or a leak in the radiator can all prevent the engine from reaching and maintaining the correct temperature. The increased airflow at highway speeds exacerbates these issues.
* Coolant Mixture: If the coolant is not a 50/50 mix of antifreeze and distilled water, it may not be able to transfer heat effectively.
* Air in the Cooling System: Air bubbles trapped in the cooling system can prevent proper coolant circulation. This is less likely to be the *sole* cause, but it could contribute to the problem.
Troubleshooting Steps:
1. Check Coolant Level: This is the absolute first thing to do. Top it off if low, but only with the correct 50/50 mix of coolant and distilled water. Do not use plain water.
2. Inspect for Leaks: Look carefully at all hoses, the radiator, and the water pump for any signs of leaks.
3. Feel the Upper and Lower Radiator Hoses: When the engine is warm (after idling for a while), feel both hoses. They should be roughly the same temperature. If the lower hose is significantly cooler, it suggests poor circulation (possibly a thermostat or water pump problem).
4. Check the Radiator Cap: Make sure the radiator cap is in good condition and properly sealed.
If you've checked these and the problem persists, it's best to take your Cadillac to a qualified mechanic. They can diagnose the issue accurately and perform the necessary repairs. Ignoring a cooling system problem can lead to serious engine damage.
