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Difficulty: MediumFuel and Air Intake Systems

An unmetered air leak develops downstream of the Mass Air Flow (MAF) sensor into the intake manifold. What effect does this intake vacuum leak have on the engine's air-fuel mixture?

  1. A
    It creates a rich mixture because the drop in intake manifold vacuum forces excess fuel through the injectors.
  2. It creates a lean mixture because extra air enters the combustion chambers without being measured by the computer.Answer
  3. C
    It prevents the intake valves from opening during the intake stroke of the four-stroke cycle.
  4. D
    It prevents coolant flow by causing the thermostat to stick in the closed position.

Answer

It creates a lean mixture because extra air enters the combustion chambers without being measured by the computer.
The Mass Air Flow (MAF) sensor measures the amount of air entering the engine so the engine control unit can inject the correct amount of fuel. When an intake leak occurs downstream of the sensor, additional 'unmetered' air enters the cylinders without extra fuel being commanded, resulting in a lean air-fuel mixture.

Step-by-Step Solution

1
Identify the role of the Mass Air Flow (MAF) sensor in fuel delivery.
The MAF sensor measures the volume and density of incoming air so the engine control unit delivers the matching quantity of fuel.
Accurate fuel calculation requires measuring all incoming intake air.
2
Analyze the impact of a vacuum leak located downstream of the MAF sensor.
Air enters the intake stream after the sensor, meaning the computer does not detect this additional air volume.
Only air passing through the sensor body is metered.
3
Determine the resulting combustion mixture ratio.
Because fuel is metered only for the measured air, the unmeasured air creates an excess of air relative to fuel, which is a lean mixture.
A lean mixture is defined by an excess of air (or deficiency of fuel) relative to the stoichiometric ratio.

Key Concept

Mass Air Flow Sensor Metering and Lean Mixture Diagnostics
Estimated Time:45s
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Topics

Fuel and Air Intake Systems

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