Exhaust Color Diagnosis

Exhaust smoke color chart.

Good combustion chambers have a tight seal. Compression can leak out if the seal is compromised, 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.

Rich Air-Fuel Ratio

Catalytic converter cutaway.

When an engine is running rich, too much fuel enters 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.

Distinguishing Between Colors

Black-colored exhaust at the tailpipe.

It can be difficult to distinguish between blue and black colored exhausts. However, a rich air-fuel ratio results in black-colored exhaust and leaves a sooty residue around the tailpipe. It also results in a negative fuel trim because the ECM is subtracting fuel from the air-fuel mixture. Oil bypassing the piston rings results in blue-colored exhaust. A wet compression test may indicate worn piston rings.