1. Verify the Thermostat Installation:
* Correct Thermostat: Double-check that you installed the *correct* thermostat for your Windstar's engine. An incorrect thermostat can cause inconsistent heating.
* Proper Installation: Ensure the thermostat was installed correctly, including the gasket (if applicable). A damaged or improperly seated gasket can cause coolant leaks and improper temperature regulation. Air bubbles trapped in the system can also cause this issue.
* Thermostat Operation: If possible, test the old thermostat in a pot of boiling water to confirm it opens properly. A faulty new thermostat is also possible, though less likely.
2. Coolant Level and Condition:
* Low Coolant: Check the coolant level in the overflow reservoir. Low coolant means the system isn't circulating enough fluid to maintain heat. Top it off with the correct 50/50 mix of coolant and distilled water.
* Contaminated Coolant: Dirty or contaminated coolant can restrict flow and reduce heating efficiency. If the coolant is rusty, murky, or has excessive sediment, it might need a flush and refill.
3. Water Pump:
* Weak Water Pump: A failing water pump might not circulate coolant effectively, leading to inconsistent heat. Listen for unusual noises (whining, grinding) from the water pump area. A visual inspection for leaks is also recommended.
4. Heater Core:
* Clogged Heater Core: A partially clogged heater core restricts hot coolant flow to the heater. This often results in inconsistent heat or weak heat. This is a more involved repair, potentially requiring a heater core replacement. Check for leaks around the heater core; this is sometimes detectable by a sweet smell of coolant inside the vehicle.
5. Radiator and Hoses:
* Restricted Radiator: A clogged radiator can impede coolant flow, resulting in poor heating.
* Clogged or Kinked Hoses: Check all hoses leading to and from the heater core and radiator for blockages or kinks that could restrict coolant flow.
6. Bleeding the Cooling System:
* Air Pockets: Air pockets trapped in the cooling system can prevent proper circulation and lead to inconsistent heating. Consult your Windstar's repair manual for the proper procedure to bleed the system. This often involves opening a bleed valve (if present) on the engine or heater core.
7. Blending Door Actuator (HVAC System):
* Malfunctioning Actuator: The blend door actuator controls the mix of hot and cold air. A faulty actuator can cause the system to deliver inconsistent temperatures. This would likely involve more constant, alternating warm and cold, not just a shift towards cold.
Troubleshooting Steps:
1. Start with the easiest checks (coolant level, thermostat installation).
2. Listen for unusual engine noises (water pump).
3. Visually inspect all hoses and connections for leaks.
4. If you're not comfortable working on your vehicle's cooling system, take it to a qualified mechanic. A misdiagnosed cooling system issue can lead to significant engine damage.
Remember to always allow the engine to cool down completely before working on the cooling system to avoid burns. Consult your Ford Windstar's repair manual for specific procedures and diagrams.
