Heater core is bad

Maybe the heater core is plugged or you haven't bled it properly?
If the heater hose returning from the core to the water pump is NOT hot, while the hose going to the heater core from the thermostat area IS hot, then the heater core is plugged probably.
You need to remove the heater hose while cool,from the thermostat area and remove the hose from the water pump area (that's the outlet of the flow thru heater core) AND THEN take a garden hose with p-is-tol grip type sprayer,and blow water back thru the outlet hose out the inlet hose. This will blow any bad plugging material out the way it got in.
Don't apply too much pressure if it is plugged bad. The heater core isn't made to stand 50lbs. water pressure. don-ohio (:^)

Either the heater core is clogged or busted, or a blend door is broken.
YOU TUBE has a lot of videos on this subject.

Is the core actually what is leaking or is it maybe your lines or connections? Also, where did you get the core? If you have changed it twice i would venture to say that the original was the first change and a replacement was the second change? Maybe you just got a bad core and it is nothing you did wrong.

If the heater core bypass refers to the water flow to the heater core, I would wonder why you did not flush the heater core as part of the system. You can drain water from engine, disconnect heater hoses and add a short hose to the heater and force a garden hose to flush the core.

I have not looked up the particulars for which heater system is on your car. Some just use duct-work to close off air flow through the heater core and the heater core always has hot water in it. Others will use a valve to shut off the water flow through the heater core and they can be either electric or vacuum operated. Combination systems are the third system which controls water flow and duct air flow at the same time.

Some of the Premium Climate control systems with Electronic displays have a computer diagnostic system which self-diagnoses problems but you need the sequence codes and explanation of where the bad part is located.

There is a youtube video called short cut to heater core, its not a bad job and the wwhole dash doesn't have to come off, just gets lifted and you get to the core. Hope you can find the vid

need to replace heater core

ok, there are a couple of things you need to check. first, for you inlet hose(from engine to heater core) is a valve, that allows hot coolant to flow to the heater core. These often get stuck or corroded on the inside. You get one at the auto parts store, and change it yourself in about 10 minutes. If this does not work, I'm sorry to say you will have to change the heater core.

Bad heater core. Replace it.

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.

when your heater core goes bad, generally it leaks inside the car and can leave a odor of antifreeze. It can effect colling of the engine...so I would change it.

It is behind the dash..but not a huge deal..take your time some saturday afternoon & youll have it