Coolant leaking at back of engine,could it be the heater core

It is likely that the heater core has sprung a leak, or the hoses leading to the heater core have failed. This would cause a loss of coolant that would make the coolant temperature sensor think it wasn't necessary to turn the fan on, and would cause steam to appear to rise out of the back of the engine. If you turn the heater on inside the passenger compartment and get steam through the vents, it's DEFINITELY a heater core breach. Otherwise, check the hoses leading to the heater core and make sure they are still connected and not leaking. Good luck!

May not be heater core, sounds like leaking water into car from some other source.Low coolant could be engine gasket related Check for coolant leaks in engine compartment & under car.
Also check for water leak at winshield before replacing heater core.

Yes it can, the coolant leaks from the heater core, and causes the engine to lose coolant. The sad part is, if your heater core in underneath your dashboard, you will have coolant in the floorboard.

Your heater core has developed a leak - the wetness on the passenger side carpet is engine coolant
from a heater core leak, or a leak in one of the two hoses connected to the back of the heater core.
Your strategy: disassemble dash down to heater core, determine whether you'll need a new heater
core or new hose (or new hose clamp), then replace the offending part.

This is usually a sign of a heater core that has a leak. check for low coolant level in your radiator. there is a condensate drain for the blower box, see if coolant is dripping from the drain under the truck. To make a temporary repair, after confirming leaking heater core, loop one of the heater core hoses back to the engine, bypassing the heater core. I say after confirming because the heater core hose nipples break easily when removing hoses. The smell in the cab would be a sweet smell from leaking coolant.

On a very humid day fog can be seen coming out of vents when using the AC. This would be a normal condition.

The problem is the heater core leaking behind the dashboard. Heater Core Removal & Installation 1997-2002 To Remove:
NOTE: Heater core must be vacuum leak tested before it is remove from vehicle if a heater core leak is suspected.

1. Drain engine coolant.
2. Remove instrument panel.
3. For 5.4L (4V), if quipped, remove junction splash shield.
4. For 5.4L (4V), if quipped, disconnect starter relay cable.
   
   
5. For 5.4L (4V), if quipped, remove junction block bracket.
6. Clamp inlet and outlet heater hoses.
7. Disconnect heater hose from heater core by gently pressing it toward heater core.
8. Remove A/C plenum demister adapter.
9. Disconnect vacuum line.
10. Remove 13 plenum chamber top screws.
11. Remove blend door assembly from case.
12. Remove heater core.

To Install:
NOTE: If needed, clean and lubricate coolant hoses with plain water.

1. Install heater core.
2. Install blend door assembly to case.
3. Install 13 plenum chamber top screws.
4. Connect vacuum line.
5. Install A/C plenum demister adapter.
6. Connect heater hose to heater core.
7. Release all clamps from inlet and outlet heater hoses.
8. For 5.4L (4V), if quipped, install junction block bracket.
9. For 5.4L (4V), if quipped, connect starter relay cable.
10. For 5.4L (4V), if quipped, install junction splash shield.
11. Install instrument panel.
12. Refill engine coolant.

Sounds like one of two things are happening. Either the hose going to the heater core (on the engine side of the dash) is leaking causing coolant to spray into the passenger compartment or your heater core is leaking which is causing the dripping from the passenger side of the dash. This would also cause a loss of coolant in the engine which would cause the engine to overheat and not enough coolant to get to the heater core to supply heat to the passenger compartment. Check your coolant level (while engine is cold) and refill it with a 50/50 mix of coolant and water after you tighten the leaking hose or replace the heater core.Then start the engine and let it warm up to operating temperature. Again let it cool down and then top off the radiator with the 50/50 mix of coolant. Do not under any circumstances open the radiator cap while the engine is hot.

Firsdt check the radiator cap thats it tight , then check for any anti freeze visible on the engine, then check under the car for any leaks. Is the smell of anti freeze inside the car ? , check the heater core you may have a leak there. the newer heater cores are aluminum and certain anti freezes, are extra acidic , which causes the anti freeze to eat at the heater core causing leaks, it maybe be a pin hole leak in the heater core ,causing the smell.

check closer to just underneath the firewall--it could be a smaller problem than a heater core--Ford 4.6--5.4 engines have a tube just underneath the intake manifold that the rubber hose goes bad and it will leak towards the back of the engine.Lets hope so-what i hear its very expensive to change a heater core in an Expedition.

First of all ensure that your car is leaking coolant fluid, and what is dripping is not just water condensation from the air conditioner.
If you have a coolant liquid leak, it can be from one of the ducts leading to radiator , from water pump or radiator itself. Check liquid levels, and if you have a leak have the car inspected by a qualified mechanic.
A broken water pump, for example, can cause your motor to blow in few miles.