* Insufficient Airflow at Low Speeds: The Corvette's cooling system relies on airflow through the radiator to dissipate heat. At freeway speeds, the car's forward momentum forces a significant amount of air through the radiator, efficiently cooling the engine. In town, speeds are much lower, resulting in less airflow and reduced cooling capacity. This is especially true if the car has a restrictive front air dam or damaged cooling shrouds that disrupt airflow.
* Low-Speed Fan Operation: The electric cooling fan(s) are designed to kick in when airflow isn't sufficient, typically at low speeds or during idling. If the fan isn't working properly (weak motor, bad relay, faulty temperature sensor), it won't provide adequate cooling in town. Even if it works, it might not be powerful enough to compensate for completely inadequate airflow.
* Radiator Issues: A clogged radiator (from debris, corrosion, or internal blockage) restricts coolant flow and heat dissipation. The increased airflow at freeway speeds might still be enough to overcome a partially clogged radiator, but at lower speeds, the effect becomes much more pronounced.
* Thermostat Problems: A stuck-closed thermostat prevents coolant from circulating until a certain temperature is reached. This could lead to overheating, especially in stop-and-go traffic where the engine doesn't reach a high enough operating temperature to open the thermostat fully.
* Coolant Leaks: A small coolant leak might not be noticeable at higher speeds because the engine heats up more slowly, but it will become a problem at low speeds where heat is building up more quickly.
* Water Pump Issues: A weak or failing water pump might not circulate coolant efficiently enough at low RPMs. While freeway speeds might still provide enough pressure to keep things moving, city driving will expose the deficiency.
* Head Gasket Failure (Less Likely): While less likely to only exhibit symptoms at low speeds, a head gasket leak could cause overheating. However, other symptoms (white smoke from exhaust, loss of coolant, milky oil) would usually be present.
In short: The difference in behavior between freeway and city driving strongly suggests a problem with airflow, fan operation, or a partially restricted cooling system. A proper diagnosis requires checking all of the above points.
