Antifreeze LEAK

If you are not seeing an external leak, or signs on the ground, suspect an internal leak. With the rate that you are consuming it, it should be obvious if it is external. Do you have misfires after the vehicle sits, and is first started, rough run that goes away in a few seconds/minutes?

Coolant consumption, in conjunction with the rough run at first start or start after soak, is probable cause to indicate an internal leak. What happens is after the engine is shut down warm, pressure exists in the cooling system. The pressure then bleeds into the unpressurized cylinder(s). The result is coolant in the cylinder which is not combustible. The most common cause for this is leaking head gaskets. The leaking head gasket is an underlying effect of failed head bolt threads. If you were to have the cooling system checked for the presence of combustion gases, you would probably get a positive result.