* Low Coolant: The most common cause. Insufficient coolant means less heat transfer to the heater core. Check the coolant level in the overflow reservoir (when the engine is cool!). If low, you'll need to add coolant (a 50/50 mix of antifreeze and distilled water is crucial). A leak somewhere in the system could be the underlying reason for low coolant.
* Thermostat Stuck Closed (Partially or Fully): The thermostat regulates coolant flow. If it's stuck closed or partially closed, the engine won't reach optimal operating temperature, resulting in less heat for the heater core. At higher speeds, the engine might be generating enough heat to overcome this partially, but not fully.
* Worn Water Pump: A failing water pump may not circulate coolant efficiently enough, leading to inadequate heating, particularly at lower speeds where engine heat generation is less. A weak pump might manage at higher speeds due to increased engine heat.
* Clogged Heater Core: The heater core itself might be partially or fully clogged with debris or rust, restricting coolant flow and reducing heat output. This is less likely to be speed dependent, but it's a possibility.
* Blend Door Actuator Issue: While less likely to cause *only* warm air, a malfunctioning blend door actuator (controls the mix of hot and cold air) could be contributing. If it's not fully opening the hot air, you'll only get warm air.
* Air in the Cooling System: Air bubbles in the cooling system can impede proper coolant circulation and heat transfer. Bleeding the cooling system (removing trapped air) might resolve this.
Troubleshooting Steps:
1. Check Coolant Level: This is the absolute first thing to do.
2. Check for Leaks: Inspect hoses, radiator, and water pump for any signs of leaks.
3. Feel the Heater Hoses: When the engine is warmed up, feel the two heater hoses (one going to the heater core, one coming from it). They should both be hot. If one is significantly cooler than the other, there's a blockage or flow problem in that section of the system.
4. Replace the Thermostat: This is relatively inexpensive and easy to replace. A new thermostat is a good preventative measure even if it's not the sole cause.
5. Bleed the Cooling System: Consult your owner's manual for the procedure.
6. Consider a professional inspection: If the above steps don't resolve the issue, it's best to take your van to a mechanic for a proper diagnosis. They can check the water pump, heater core, and other components more thoroughly.
Remember to always let the engine cool down completely before working on any part of the cooling system to avoid burns. Use caution when handling coolant, as it is toxic.
