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Difficulty: HardFuel and Air Intake Systems

An engine hesitates during acceleration due to a cracked vacuum hose downstream of the mass airflow (MAF) sensor. What air-fuel mixture condition does this unmetered air leak cause?

  1. A
    A rich mixture, because the unmetered air increases intake manifold pressure and commands extra fuel injection
  2. A lean mixture, because extra air enters the intake manifold without being measured by the sensor for fuel calculationAnswer
  3. C
    A rich mixture, because the engine control module senses the pressure change and defaults to open-loop enrichment
  4. D
    An interrupted intake stroke, because atmospheric pressure entering the leak stalls piston downward movement

Answer

A lean mixture, because extra air enters the intake manifold without being measured by the sensor for fuel calculation
In an electronic fuel injection system, the engine computer calculates fuel delivery based on air mass reported by the mass airflow (MAF) sensor. An air leak located downstream of the sensor introduces 'false' or unmetered air directly into the intake manifold. Because the computer does not know this extra air exists, it delivers fuel only for the measured quantity, creating a lean mixture (excess air, insufficient fuel) that leads to hesitation.

Step-by-Step Solution

1
Analyze sensor location and air measurement function
The mass airflow (MAF) sensor measures only the volume and mass of air passing directly through its housing.
The engine control computer relies on MAF readings to calculate the precise pulse width for the fuel injectors.
2
Determine the impact of downstream air entry
A cracked vacuum hose allows outside 'unmetered' air to enter the intake manifold after the MAF sensor.
This unmetered air bypasses the sensor, so the computer has no record of this additional incoming air.
3
Evaluate the resulting air-to-fuel ratio
More air enters the combustion chamber than the computer supplied fuel for, creating an excess of air relative to fuel.
An excess of air (or deficiency of fuel) relative to the stoichiometric ideal creates a lean condition, leading to stumbling and hesitation.

Key Concept

Unmetered Intake Air and Air-Fuel Mixture Imbalance
Estimated Time:1m 30s
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Topics

Fuel and Air Intake Systems

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