I just acquired a 1999 Grand Prix. Engine Hydro Locked. My immediate thought was Head gasket. I pulled the Intake and rear Head off and couldn't find a distinctive crack. When I removed the upper plenum i found the lower manifold filled with coolant. I inspected the head and cylinder and found no distinctive cracks. The car has been well maintained. I am puzzled. I dont want to waste the cask on a gasket set to find out it is something else.

Any Ideas.