I have a 2000 Ford Ranger with a new t-stat and recently new coolant. I was having overheating problems hence the new t-stat. Now my truck runs in the normal on the surface streets but runs cool on the freeway. The heater blows warm air on the freeway but it is not the warmest of air. There shouldn't be any air in the system. Any suggestions before ripping into the truck to replace the heater core?

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2000 Ford Ranger XLT V-6 3.0L