1991 Other Ford Models catalytic converter

The engine is running way too rich and the excess fuel is being combusted in the Cat this can be caused by an exhaust leak, the leak will cause the air/fuel mixture to become rich, the extra fuel can't be used by the engine so it puts it into the exhaust system the extra air pulled in from the exhaust leak and the heat creates the combustion of fuel in the Cat. The oxygen sensor sees a lean condition and commands the computer to go full rich, this combination can cause the condition you have. Test the oxygen sensor/s whether its switching properly from rich to lean and from lean to rich-