Rich Air-Fuel Ratio
When an engine is running rich, too much fuel is entering the combustion chamber. This rich air-fuel ratio results in high HC's and black sooty deposits around the tailpipe. Catalytic converters can overheat from the excessive hydrocarbons (HC's) in the exhaust stream.
The combustion chambers are sealed tight. When this seal is broken, or if the valve train is out of geometry, a leak can occur. Compression can leak out, and fluids like coolant and engine oil can leak in. An initial visual inspection of the exhaust color can point a technician in the right direction.