Hello, I have a 97 f150 4x4, 4.6 liter. My problem that I'm having is the engine heats up fine but the cab doesn't. For example, 25 degrees outside temp. engine at normal operating temp in about 3 or 4 miles. when engine is warm i get no heat from heater, yet if i drive another 10 miles the heater will start to kick out warm air. the farther i drive the warmer the air gets coming out the vents. i have changed coolant, new themastat. flushed heater core. I'm not sure what to check. it acts like heater core is plugged but the longer the engine runs it opens up. Any help would be great. thanks