* Speed-dependent failure: The fact it only fails at higher speeds strongly suggests something related to airflow or pressure. When you're driving fast, the engine's cooling system is under greater demand and pressure.
* Heater and AC affected: Both systems rely on coolant circulation. A problem affecting one will often affect the other.
The most likely culprits are:
* Low coolant level: This is the easiest to check. Simply inspect your coolant reservoir and add coolant if needed (making sure it's the correct type for your Pontiac Grand Prix).
* Leaking coolant: A leak anywhere in the system (radiator, hoses, water pump, etc.) would cause a loss of pressure, especially at higher speeds when the system is working harder. Inspect all hoses and the radiator carefully for any signs of leaks.
* Failing water pump: A weak or failing water pump might not be able to maintain sufficient pressure at higher speeds. Listen for unusual noises coming from the water pump area.
* Clogged radiator: A clogged radiator restricts coolant flow, reducing pressure, especially under load.
* Faulty pressure cap: A bad pressure cap won't maintain the correct system pressure.
Before doing anything: Let the engine cool completely before checking coolant levels or inspecting components. Hot coolant can cause serious burns.
Recommendation: Start by checking the coolant level. If it's low, find the leak before adding more coolant. If the level is okay, a mechanic should inspect the system for leaks, pressure, and the functionality of the water pump. A pressure test of the cooling system is a good diagnostic step for a professional.
