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Difficulty: EasyExhaust and Emissions Systems

An automotive technician observes dense black smoke exiting a vehicle's tailpipe along with a strong odor of unburned gasoline. Which engine operating condition is the most likely cause of this exhaust symptom?

  1. A
    Lean air-fuel mixture
  2. Rich air-fuel mixtureAnswer
  3. C
    Stuck-closed engine thermostat
  4. D
    Weak alternator output

Answer

Rich air-fuel mixture
A rich air-fuel mixture occurs when too much gasoline is delivered into the combustion chambers relative to the intake air. Because there is not enough oxygen to burn all of the fuel completely, unburned hydrocarbons and carbon soot pass through the exhaust system, creating dense black smoke and a distinct fuel odor at the tailpipe.

Step-by-Step Solution

1
Analyze the tailpipe exhaust symptom
Dense black smoke and the odor of unburned gasoline indicate incomplete fuel combustion.
Exhaust smoke color provides key diagnostic clues about combustion conditions.
2
Determine the combustion chemistry cause
Excessive fuel entering the cylinders relative to available air creates soot (carbon) and raw fuel vapors.
When an engine runs rich, there is insufficient oxygen to fully combust all hydrocarbons into carbon dioxide and water vapor.

Key Concept

Tailpipe Exhaust Smoke Diagnosis and Air-Fuel Mixture
Estimated Time:45s
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Topics

Exhaust and Emissions Systems

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