I have a 2003 Expedition with the 5.4 2v with 235,000 miles on it. When I first got it from my dad I knew it had a coolant leak on the passenger side of the engine but couldn’t find it, eventually figured out it was one of the heater hoses because it blew out on me, but after fixing that it still seems to use coolant, granted according to the owners manual it’s a 7 gallon coolant system so I’ve never ran it out, even with the leaky heater hose. Since I fixed the heater hose I put about 2 gallons in it initially to refill the lost coolant but since then I’ve probably had to put in 2 or 3 more gallons, normally I would think I just lost all of that coolant from blowing that heater hose since it never overheats even when I beat on it and neither the front or back heater stop blowing hot air, however it seems no matter how long I run the thing it always blows steam from the exhaust pipe at idle. Every time I refill it the level is just about at the bottom of the reservoir but it never seems to drop below that point, one of my questions is is that where the level should be when hot? It seems like even when I let it cool down the level doesn’t return to the cold fill line. I guess to me it’s possible I just haven’t bled all the air from the system yet and there’s just a bunch of condensation in all 4 cats and the muffler iirc, but again the sheer amount of steam the thing puts out of the exhaust is concerning. Obviously I would smell the steam and see if it smells like antifreeze however the engine bay always smells like it since I never cleaned it out when the hose blew and my sense of smell is bad enough I have a hard time smelling gasoline so I have no clue what to think. My main question is do head gaskets ever fail on these and can the intake gasket fail in such a way that it burns coolant, there’s zero cross contamination between the oil and coolant so I wouldn’t think it’s a head gasket but I really don’t know.

 

Depending on how and where a head gasket fails, it can burn coolant. Get some clean cardboard and park your truck on it over night. In the morning, back the truck off it and look for spots. This will tell you if you have external leaks.When I ran a shop back in the 80's, I had one car come in that the owner couldn't find the leak. We looked at expansion plugs, intake gaskets and so on. It turned out to be a dry rotted hose to the expansion tank. When it warmed up, coolant went out, but the hole cause air to suck back in. Hence the radiator kept getting low.

I'd head over to the parts store and rent a head gasket testing kit. You'll have to buy the reagent. This will change color if combustion gasses are present. You can also try a radiator pressure tester but that doesn't always show head gasket leaks.

If the cardboard is clean and you can't find any leaks, it's possible you have either a blown head gasket or cracked head.