* Heater Core: The most likely culprit. The heater core is a small radiator inside the car that warms the air blown by the fan. If it's clogged, restricted, or leaking internally, it won't effectively transfer heat to the cabin air. A clogged heater core will often lead to reduced or no heat, even when the engine is hot.
* Heater Control Valve: This valve controls the flow of coolant through the heater core. If it's stuck closed, no hot coolant will reach the heater core. It's relatively inexpensive to replace and easy to access, usually located near the firewall.
* Air in the Cooling System: Even after replacing the water pump and thermostat, air pockets can remain trapped in the cooling system. These air pockets can prevent proper coolant circulation, leading to inconsistent heating. Try bleeding the cooling system thoroughly. There's usually a bleeder valve somewhere near the engine or radiator. Consult your car's repair manual for the exact location and bleeding procedure.
* Low Coolant: Check your coolant level again. Even a small leak can cause the system to lose pressure and reduce its effectiveness. Look for evidence of leaks around the radiator, hoses, water pump, and heater core.
* Coolant Mixture: Ensure you have the correct mixture of coolant and water (usually a 50/50 mix). Too much water can lower the coolant's freezing point and boiling point, affecting its efficiency in transferring heat.
* Worn out or Damaged Hoses: Check all hoses, especially those connected to the heater core. If they're cracked, pinched, or collapsing, they can restrict coolant flow.
Troubleshooting Steps:
1. Check Coolant Level: Make sure your coolant level is correct and that there are no visible leaks.
2. Bleed the Cooling System: This is crucial, as air pockets are a frequent cause of heating problems.
3. Inspect Heater Hoses: Check for kinks, collapses, or leaks in the hoses leading to and from the heater core. Feel them while the engine is running to check for temperature differences—a cold hose suggests a blockage or valve issue.
4. Test the Heater Control Valve: See if you can manually actuate the valve (if accessible). If it doesn't move freely, it needs to be replaced.
5. Consider the Heater Core: If the above steps don't resolve the issue, the heater core is the most probable remaining culprit. Replacing a heater core is a more involved job, often requiring partial disassembly of the dashboard.
If you're not comfortable performing these checks or repairs yourself, it's best to take your Sunbird to a qualified mechanic for diagnosis and repair. They'll have the tools and expertise to pinpoint the exact problem.
