* Water pump: A malfunctioning water pump won't circulate coolant effectively, even if the thermostat is working correctly. Listen for unusual noises from the pump (whining or groaning). A visual inspection might reveal leaks or a damaged impeller.
* Blend door actuator: This small motor controls the flow of air between the heater core and outside air. A faulty actuator might be stuck in the "outside air" position, preventing hot coolant from warming the air blowing into the cabin. This is a common problem on Grand Cherokees. You'll need to check your blend door operation either manually or electronically, depending on your model year.
* Heater core: The heater core is a small radiator located inside the passenger compartment that uses hot coolant to warm the air. It could be clogged, leaking (causing a sweet smell inside the car), or simply failing. A leak is usually easy to spot (look for wet spots on the passenger-side floor or carpet).
* Coolant level (again): Even if the reservoir looks full, air pockets can form in the system preventing proper circulation. Make sure you properly bled the air from the cooling system *after* replacing the thermostat. There's usually a bleed screw somewhere on the engine or radiator. Refer to your owner's manual for the correct procedure.
* Radiator cap: A faulty radiator cap won't maintain the proper system pressure, hindering effective coolant circulation and heat transfer.
* Wiring/fuses: A blown fuse or faulty wiring to the heater motor or the blend door actuator could prevent the system from working. Check your fuse box.
Troubleshooting Steps:
1. Check for air pockets: Bleed the cooling system thoroughly.
2. Listen for water pump noises: A failing water pump will be noisy.
3. Inspect the heater core for leaks: Look for wetness under the carpet on the passenger side. Smell the air – a sweet odor suggests coolant leakage.
4. Test the blend door actuator: This might require accessing the actuator and testing its movement.
5. Check fuses and wiring: Examine the fuses and wiring related to the heater system.
If you're not comfortable tackling this yourself, take it to a mechanic. Diagnosing heating problems can be tricky, and it's better to get a professional opinion before replacing parts unnecessarily. Provide the mechanic with the information that you've already replaced the thermostat.
