i have a 93' f150 I6, i got the truck in feb.09 and ever since ive had it the temp would run cold below normal, i figured it was my thermostat and didnt worry about it because summer was coming up, now it is getting cold and i just put a new thermostat in it, that fixed that problem but my heater doesnt get hot, all the water lines running to the heater core are hot, ive read something about a blend door, could this be my problem?