-
If you lost almost all the coolant in 24 hours, and don’t see a puddle, it’s not leaking it. That’s a lot of coolant to just disappear. If it was leaking you’d see at least a drip somewhere, more likely a river.
-
Pretty sure that when the intake manifold gasket goes you can see the coolant on top of the motor, in the little recesses around where the intake meets the block. If that assumption is right, and you don’t see coolant, it’s not that.
-
The heater core can leak too, but again, if it’s a leak you’d see it.
-
That leaves head gasket. Burning that much you should be able to see white smoke coming out of the exhaust, or coolant in the oil. Drain the oil first and see what you have. Then test the compression on each cylinder. they should all be within about 5psi of each other. I’m guessing you’ll find one or two that are considerably different than the rest, and that’s where your head gasket is blown.