* Blend door actuator: This controls the mix of hot and cold air. A faulty actuator might be stuck in the cold position. This is a common problem on Grand Caravans of that era.
* Radiator coolant level: Low coolant will prevent the heater core from getting hot enough. Check your coolant level and top it off if necessary (using the correct 50/50 mix of coolant and distilled water). Also, look for leaks.
* Heater core: The heater core itself could be clogged, leaking, or failing. This requires more involved repair or replacement.
* Water pump: A failing water pump won't circulate coolant effectively, limiting heat to the heater core.
* Thermostat installation: While you've changed the thermostat, it's possible it wasn't installed correctly. Air pockets in the system can also hinder proper heat transfer. Ensure the system is properly bled of air after installation. A faulty new thermostat is also possible, though less likely.
* Wiring or fuses: Check the fuses and wiring related to the heater system.
* Vacuum lines: Some aspects of the climate control system are controlled by vacuum lines. Check for leaks or damage.
To diagnose the problem further, you might:
* Check for hot coolant hoses: Feel the hoses leading to and from the heater core. If they're both cold, the problem likely lies further upstream (water pump, thermostat, coolant level). If one is hot and the other is cold, the problem is likely within the heater core or its connections.
* Listen for unusual noises: Unusual gurgling or whistling sounds from the heater core area might indicate a problem.
* Consult a repair manual: A repair manual specific to your 1999 Dodge Grand Caravan will provide detailed diagrams and troubleshooting steps.
If you're not comfortable working on your vehicle yourself, it's best to take it to a qualified mechanic for diagnosis and repair.
