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Difficulty: HardExhaust and Emissions Systems

During a vehicle emissions inspection, tailpipe measurements indicate excessively high levels of oxides of nitrogen (NOxNO_x) during an acceleration test, while hydrocarbon (HCHC) and carbon monoxide (COCO) concentrations remain within normal limits. Under moderate acceleration, the engine also exhibits audible spark knock (detonation). Which of the following component malfunctions is the most likely cause of these symptoms?

  1. A
    An evaporative emission (EVAP) canister purge solenoid stuck in the open position
  2. An exhaust gas recirculation (EGR) valve stuck in the closed positionAnswer
  3. C
    A positive crankcase ventilation (PCV) valve stuck in the wide-open position
  4. D
    An engine cooling system thermostat stuck in the open position

Answer

An exhaust gas recirculation (EGR) valve stuck in the closed position
The exhaust gas recirculation (EGR) valve is designed to deliver a controlled amount of inert exhaust gas into the intake air-fuel mixture during acceleration and cruise conditions. This inert gas absorbs combustion heat, keeping peak combustion chamber temperatures below 2,500∘F2,500^\circ\text{F} (1,370∘C1,370^\circ\text{C}), where atmospheric nitrogen and oxygen chemically combine into NOxNO_x. When the EGR valve fails to open, combustion temperatures surge under load, causing excessive NOxNO_x output and pre-ignition or detonation (spark knock).

Step-by-Step Solution

1
Analyze the emission readings and physical engine symptoms.
The engine produces high levels of oxides of nitrogen (NOxNO_x) and exhibits detonation (spark knock) under load, while hydrocarbons (HCHC) and carbon monoxide (COCO) are normal.
Oxides of nitrogen form primarily when peak combustion chamber temperatures exceed approximately 2,500∘F2,500^\circ\text{F} (1,370∘C1,370^\circ\text{C}), which is the same condition that triggers spark knock.
2
Identify the emission control system responsible for peak combustion temperature regulation.
The Exhaust Gas Recirculation (EGR) system meters small quantities of inert exhaust gas back into the intake manifold during acceleration and cruising.
Inert exhaust gas dilutes the incoming air-fuel charge and absorbs heat, maintaining combustion temperatures below the 2,500∘F2,500^\circ\text{F} threshold required for NOxNO_x formation.
3
Evaluate the failure mode of a stuck-closed EGR valve.
Without exhaust gas dilution, in-cylinder temperatures spike under load, generating high NOxNO_x and causing detonation.
Because the baseline air-fuel mixture calibration is undisturbed, HCHC and COCO remain within normal limits while NOxNO_x surges.

Key Concept

The Exhaust Gas Recirculation (EGR) valve meters inert exhaust gas into the cylinders to lower peak combustion temperatures, preventing the formation of oxides of nitrogen (NOxNO_x) and suppressing spark knock.
Estimated Time:1m 30s
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Topics

Exhaust and Emissions Systems

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