Troubleshooting by Exhaust Color

This question deals with troubleshooting the condition of the combustion chamber by exhaust color. The combustion chamber requires the correct air fuel mixture as well as a tight seal for proper combustion. A rich fuel mixture will result in black smoke emitting from the tailpipe. This is caused by excessive hydrocarbon or unburned fuel in the chamber. Leaking valve seals or weak piston rings result in blue gray smoke. An internal coolant leak like a faulty head gasket will result in white smoke.

Troubleshooting by Exhaust Color

Blue/Gray Smoke: Blue gray smoke is an indication of oil burning in the combustion chamber. These are possible symptoms and causes:

Valve Seals: Leaking valve seals will cause blue/gray smoke at startup, because gravity pulls oil past the seals into the cylinder after the engine is shut down.

Valve Guides: Excessive clearance between the valve stem and the valve guide will allow oil to leak past the gap into the cylinder.

Piston Rings: Worn or damaged piston rings will cause oil blowby resulting in blue/gray smoke.

Worn Cylinder Walls: Worn cylinder walls cause oil blowby resulting in blue/gray smoke.

PCV System: A stuck closed PCV system will cause excessive crankcase pressure resulting in blue/gray smoke.

Black Smoke: Black exhaust smoke is an indication of a rich fuel condition. These are possible causes:

Fuel Injectors: A leaking or dripping fuel injector will cause a rich fuel condition.

Fuel Pressure Regulator: A stuck closed fuel pressure regulator will cause a rich fuel condition.

Fuel Return: A restricted fuel return line will cause a rich fuel condition.

White/Gray Smoke: White exhaust smoke is an indication that coolant is burning in the combustion chamber. These are possible causes:

Cylinder Head: A crack in the cylinder head (around the coolant jacket) will cause coolant to enter the combustion chamber.

Engine Block: A crack in the deck of an engine block near a coolant jacket will cause coolant to enter the combustion chamber.

Head Gasket: A damaged or blown head gasket will cause coolant to enter the combustion chamber resulting in white/gray smoke at the tailpipe.