i kinda got the same problem you do . my lower intake gasket leaks right around the overhang on the intake and the head on the front of the engine.first off it started leaking back in the winter with a noteable leak under the truck ( just a drop or two) but when summer rolled around it does not leak any drops on the ground but still use's coolant ( i keep it about a inch above the fill line on the overflow tank) ....so i'm thinking that the leak is there but it evaporates so quicklly it does not have time to hit the ground.
but yours ...no leaks anywhere and your still lossing coolant then i would do what north/west said and do a compression test because it all indicates a head gasket problem and not the intake .i could be wrong and the lower intake is leaking into the combustion chamber but like mine and the others i have talked to about this ( intake gasket failure) they all leak outward .
mines got 200486 miles on it and lucklly i have not had head gasket problems ,how many miles on yours ....over 200k ...may that head gasket .i noticed that from the people i have talked to about these issues have almost gone out or start to fail about the same time for different year makes so they all have about he same life span so to speak .
i would take the plugs out one by one till you find one that is different than the others ( this may point the direction in which to look and may save you some time in figureing out whats going on ( example: like if you find that the plug on the drivers side on the front of the engine is discolored then this may indicate the lower intake gasket (( this is right near the common problem area )) .........doing it this way may save you the time and money in tearing something apart that did'nt need replaceing ...but does now ! it's free anyways .
hope this helps . . .
