1998 mercury mountaineer problem

I have a 98 mountaineer with the 5.0 302 in it that i bought 2 weeks ago. When i use the heat only cold air blows out sometimes for awhile and regardless of what setting i put it on (floor, vent, defrost, etc.) it will still blow cold air. Sometimes I will hear a "pop" noise in the dash and the heat will start working. Is something messed up with my blend door? Help would be greatly appreciated guys. thank you.

Have you done a self test on the eatc? If that test passes, and you're sure that the coolant level is correct and the thermostat isn't sticking, check the vacuum controlled valve that's inline on the in heater hose. It's located just before the firewall, right next to the ac/blower motor plenum. When your running the ac, this valve closes and stops the hot coolant from entering the heater core/plenum.