Free A.S.E. Study Guide
An ASE Study Guide for the Automotive Technician
Exhaust Color Diagnosis
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Blue/Gray Smoke: Bluish gray exhaust is an indication of oil burning in the combustion
chamber. These are possible causes:

    o        Valve Seals: Leaking valve seals will cause blue/gray smoke at startup
    because oil leaks past the seals into the cylinder after the engine turned off.
    o        Valve Guides: Excessive clearance between the valve stem and the valve
    guide will allow oil to leak past the gap into the cylinder.
    o        Piston Rings: Worn or damaged piston rings will cause blow by resulting in
    blue/gray smoke.
    o        Worn Cylinder Walls:  Worn cylinder walls cause blow by resulting in
    blue/gray smoke.
    o        PCV System: A stuck closed PCV system will cause excessive crankcase
    pressure resulting in blue/gray smoke exiting the tailpipe.


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

    o        Fuel Injectors: A leaking fuel injector will cause a rich fuel condition.
    o        Fuel Pressure Regulator: A stuck closed fuel pressure regulator will cause
    a rich fuel condition.
    o        A restricted fuel return line will cause a rich fuel condition.


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

    o        Cylinder Head: A crack in the cylinder head (around the coolant jacket) will
    cause coolant to enter the combustion chamber.
    o        Engine Block: A crack in the deck of an engine block near the coolant
    jacket will cause coolant to enter the combustion chamber.
    o        Head Gasket: A damaged or blown head gasket will cause coolant to enter
    the combustion chamber resulting in white/gray smoke coming from the tailpipe.