* Thermostat Stuck Open (Less Likely): While a stuck-open thermostat would prevent the engine from reaching operating temperature quickly, it shouldn't affect the heater *after* the engine is warm. Your description suggests the engine *does* reach operating temperature. This is less likely but still possible if there's a partial blockage.
* Water Pump Issues: This is a strong possibility. The water pump circulates coolant throughout the engine and heater core. If the pump is weak or failing, it might not circulate enough coolant when under load (driving), leading to insufficient heat in the heater core. A failing water pump often makes a whining or groaning noise. Listen carefully.
* Heater Core Problems: The heater core itself could be clogged, partially blocked, or leaking internally. A blockage prevents coolant from flowing through it, resulting in no heat. A leak wouldn't necessarily show itself externally, but would cause low coolant levels over time.
* Low Coolant: Check your coolant level immediately. Low coolant means the system can't transfer enough heat. A leak somewhere in the system (radiator, hoses, heater core) could be the cause.
* Air in the Cooling System: Air trapped in the cooling system can impede coolant flow, especially to the heater core. Properly bleeding the cooling system is essential to ensure proper circulation. This often involves opening bleed valves at specific points in the system (consult your repair manual).
* Coolant Flow Restrictions: Something could be partially blocking coolant flow somewhere in the system – a partially clogged hose, a kinked hose, or even debris inside the radiator.
Troubleshooting Steps:
1. Check Coolant Level: The most important first step. Add coolant if low, but be cautious as adding coolant to a hot engine can be dangerous.
2. Listen for Water Pump Noise: Check for unusual noises from the water pump area while the engine is running.
3. Feel the Heater Hoses: When the engine is warm, feel the two heater hoses (one going to the heater core and one returning from it). They should both be approximately the same temperature and significantly warm. If one is significantly cooler than the other, there's a problem with flow through the heater core.
4. Bleed the Cooling System (If Applicable): Your Acura Integra may have a bleed valve. Consult your repair manual or a reliable online resource for your specific model to find the location and bleeding procedure.
5. Inspect Hoses and Connections: Visually inspect all hoses for cracks, kinks, or looseness.
If the problem persists after these steps, you'll likely need to take your car to a mechanic. They can diagnose the specific cause and perform the necessary repairs, which may include replacing the water pump, heater core, or addressing other cooling system components. Don't continue driving with a potentially faulty cooling system, as overheating can cause serious engine damage.
