1. Cooling System Integrity:
* Air in the system: Even after a thermostat change, air pockets can prevent proper coolant circulation. Make sure you properly bled the cooling system after the thermostat replacement. There's usually a bleeder valve somewhere on the engine or radiator; consult your repair manual for the exact location and procedure. Sometimes, running the engine with the heater on high for a while can help purge air.
* Low coolant level: Check the coolant level in the overflow reservoir. If it's low, you have a leak somewhere in the system that needs to be addressed. Top it off with the correct 50/50 mix of coolant and distilled water (never just water).
* Coolant leaks: Carefully inspect all hoses, clamps, the radiator, and the water pump for any signs of leaks (wet spots, dripping, etc.).
* Radiator cap: A faulty radiator cap can prevent the system from building sufficient pressure, impacting coolant flow. Replace it if it's suspect.
* Water pump: A failing water pump won't circulate coolant effectively. You might hear unusual noises from the water pump (whining, grinding) if it's failing. Check for leaks around the water pump as well.
2. Heater Core:
* Clogged heater core: The heater core is a small radiator inside the car's dash that warms the air. Over time, it can become clogged with debris or rust, restricting coolant flow. This is a more involved repair, requiring accessing the heater core often through the dash.
* Heater core leak: A leak in the heater core will result in coolant leaking into the car's interior. You might notice a sweet smell (like antifreeze) or even wet carpeting. This also needs repair or replacement.
3. Heater Control System:
* Blend door actuator: This is a small motor that controls the mix of hot and cold air. A faulty actuator might be stuck in the cold position. You might hear clicking or grinding sounds from the dash area when adjusting the temperature controls.
* Heater control valve (or blend door): This valve regulates the flow of coolant to the heater core. It could be stuck closed.
4. Thermostat (again):
* Incorrect thermostat: Ensure you installed the correct thermostat for your 1995 Grand Marquis 4.6L engine. Using the wrong thermostat can cause problems.
* Faulty thermostat (despite new one): Though unlikely if you just replaced it, there's a small chance the new thermostat is defective.
Troubleshooting Steps:
1. Check coolant level and look for leaks. This is the most important first step.
2. Bleed the cooling system. This is crucial after any work on the cooling system.
3. Check the radiator cap. Replace if needed.
4. Listen carefully near the heater core for unusual noises when the heater is on. This can help diagnose a blend door or actuator issue.
5. Verify the heater control valve operation (if accessible).
6. If you're comfortable, feel the heater hoses. With the engine running and the heater on, the hose going *into* the heater core should be hot, and the hose coming *out* should be somewhat warmer (but not as hot as the input hose). If both are cold or only slightly warm, there's a restriction somewhere in the system (likely the heater core or a valve).
If you're not comfortable tackling these repairs yourself, take your Grand Marquis to a qualified mechanic for diagnosis and repair. A proper diagnosis can save you time and money in the long run. It's important to address any cooling system problems quickly to prevent engine damage.
