This could be a problem with coolant getting past the head gasket into the combustion cylinder. Try taking out all of the spark plugs and unplugging the fuel pump relay. Then crank the engine over and see if any liquid is coming out of the plugholes; you can hold up a sheet of cardboard to the holes to see if anything is coming out.
If there is coolant getting into the combustion chamber, then the cylinder head will need to be removed and the head gasket replaced. Depending on whether or not there has been any overheating, the cylinder head may need to be machined to ensure a flat sealing surface.