Luke warm heat in a 2002 expediton
Start with Flushing out your cooling system. Flush it out with Preston Radiator flush, you have your preference of ether the super flush which takes 3-6 hours or the 10 min flush which is good if you don't have the time to flush out your cooling system.
After you have flushed out your cooling system, replace your thermostat and you should be good for another cold winter driving. What happens is that after a few years of the coolant not being serviced, the coolant crystallizes and clogs up the heater core and radiator causing poor circulation. Poor circulation will cause poor heat and or overheating issues to the engine.
Before you go through all that inspect to make sure you have the right amount of coolant in your cooling system.
Good luck and thank you for using fixya
Can you feel heater hoses when engine warmed up, where they go into the firewall to attach to heater core from engine? If one hot & other quite a bit cooler, there's something restricting the flow of coolant. If both pretty close to same temperature, then blend air door in duct work under dash or wherever located on your particular vehicle, is probably stuck or not functioning properly.
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