I have a 2002 expedition, just bought it in July. This past weekend while driving in the mountains I noticed my heater is blowing lukewarm air. The fan and door actuator seem to be functioning properly, the air is directed to the proper vents, and the fan blows hard.

The guy I bought the truck from (trust worthy) said that he was never happy with the heaters performance. He had the thermostat replaced and the both heater cores flushed this past winter. That made things better, but obviously not fixed.

When the truck has been running for a while, and I turn the heat on and up, it'll blow hot as you please for about ten seconds, then goes cool.

There is no coolant leaking in the truck, no scent of coolant, no oily mist coming from the defrost vents, just cool air from the front and back zones.

Could it be the temperature control, or am I going to have to replace my heater cores?

Any help trouble shooting would be appreciated. Thanks.

 

That's a sign of restricted coolant flow through the heater core(s). If a flush doesn't solve the problem then the cores need to be replaced or it's also possible the water pump impellers have corroded and the pump isn't moving enough coolant. But such an issue with the pump also tends to cause the engine to run hotter, especially in the heat and under heavier loads.

 

That or the hot water valve there at the firewall malfunctioning. Shoot the water lines on both sides of the hot water valve with a IR temp gun when this is happening. See if the hose on the outlet side is noticably cooler. The vacuum diaphragm inside the valve goes bad.


-Mike






Hot Water Valve

 

The default on the heater valve is open. So if the diaphragm fails the coolant will still flow.