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Difficulty: MediumFuel and Air Intake Systems

A technician observes black smoke exiting the exhaust tailpipe along with carbon-fouled spark plugs. Which fuel system malfunction is the most likely cause?

  1. A
    A restricted fuel filter limiting fuel delivery to the fuel rail
  2. B
    A ruptured vacuum hose allowing unmetered air into the intake manifold
  3. A leaking fuel injector delivering excessive gasoline into the cylinderAnswer
  4. D
    An exhaust valve failing to open during the engine intake stroke

Answer

A leaking fuel injector delivering excessive gasoline into the cylinder is the most likely cause.
Black smoke emitted from the tailpipe and heavy carbon soot on spark plugs are definitive signs of an overly rich air-fuel mixture. A leaking or stuck-open fuel injector continuously drips or sprays excess fuel into the combustion chamber, exceeding the stoichiometric ratio and leaving unburned carbon.

Step-by-Step Solution

1
Analyze the observed engine symptoms of black tailpipe smoke and dry, black carbon deposits on spark plugs.
Identify that the symptoms indicate incomplete combustion caused by an overly rich air-fuel mixture (excess fuel relative to air).
When there is too much fuel or insufficient air for complete combustion, unburned carbon is released as black smoke and soot deposits.
2
Evaluate the effect of each listed malfunction on the air-fuel ratio.
Determine that a leaking fuel injector adds excess fuel directly to the cylinder, creating the rich condition.
Restricted filters and vacuum leaks create lean mixtures (excess air/insufficient fuel), while valve timing misconceptions do not account for raw excess fuel delivery.

Key Concept

Air-fuel mixture diagnostics and symptoms of an overly rich fuel condition in internal combustion engines.
Estimated Time:1m 0s
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Topics

Fuel and Air Intake Systems

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