There are typically 2 scenarios: The first scenario is exhaust gas in the cooling system. This is typically caused by a PCC seal leak or cylinder head gasket leak. We have found major gas leaks are due to a PCC seal leak, either by PCC studs breaking or as Stephen mentioned poor sealing surface. We have also found the stiffer bronze seal rings are less forgiving to surface imperfections and don’t seal as well as the copper seal rings. These types of leaks can be found by using a jig with clear plastic tubing that attaches to each cylinder head vent tube. When you run the engine the leaking cylinder will be blowing lots of bubbles through the vent tubes.
Due to the excessive amount of exhaust gas that enters into the system you may see a drop in your coolant level, but this is due to coolant overflowing back to your maintenance tank due to being displaced by the exhaust gas in your system.
The second scenario is an exhaust seat leak. You will have glycol consumption, but little to no exhaust gas entering your coolant system. This can be found by borescoping the cylinders and looking for coolant dripping or residue on the exhaust seat pockets.