I have a 2001 expedition 4.6 liter. The air works great but when you turn on the heater it doesnt get warm. air blows out but its not warm. what is the problem? please help

At your firewall there is a inlet and a outlet for coolant to go thru your heretofore it sounds like it's plugged take the hoses off at the firewall hook up a couple how's to the inlet and outlet a water source to the inlet force clean water thru the inlet dirty water should float out of the outlet hook your hoses back up run to temperature turn your heat on

im just guessing from the flap door moving it's in the body control module (BCM) it controls heater defroster and a/c

If the heater hoses are hot, the problem may be a faulty blend door under the dash. The blend doors direct air thru the heater core and the vents.

I would make sure the core is the problem. You may have a heater control valve that cuts off flow to the core by vac. or a blend door that cuts off air flow thru the core.
You will need a shop manual to replace the core. The dash and console have to come out, along with discharging the ac.

Is the correct type of heater core that use for rear heater?Check fan fix its correct side,or wiring change or return water line is blocked.

Try turning on your heater.....If blower doesnt work on that either, then your blower motor fuse is either blown or has quit...

Notorious for plugged heater cores. Also updated plastic restriction couplers mounted at heater core available from dealer. Try back flushing heater core. Run engine till its at operating temperature then carefully hold the inlet and outlet hose breifly with heater fan on high if hoses are nearly the same temp then flow is good , if not then core is plugged.

impeller is made of plastic on teh water pump and its slipping and spinning on the shaft renew before weather warms up as engine will overheat

the blendoor on the heater box maybe broken ck lines going to heater core and make sure they are hot. does the heat temp change at all from cold to hot?

your controls have problems. the heat is there it is just not getting out because your control lines are not setting the vent controls into the proper positions. i had a chev van that lost some of the foam in side there and it blocked the whole front off. i had to rip it all apart to get at it