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Question

Difficulty: MediumExhaust and Emissions Systems

An automobile engine runs smoothly at idle but severely bogs down and loses power during high-speed highway acceleration. An exhaust backpressure gauge installed ahead of the exhaust system measures 4.2 psi4.2\text{ psi} at 2500 RPM2{}500\text{ RPM}, whereas normal backpressure is below 1.5 psi1.5\text{ psi}. Which of the following is the most likely cause of this condition?

  1. A
    A thermostat stuck in the open position preventing the engine from reaching operating temperature
  2. B
    An excessively lean air-fuel mixture delivering surplus intake air into the combustion chambers
  3. A restricted or melted ceramic honeycomb substrate inside the catalytic converterAnswer
  4. D
    An intake valve failing to close completely during the power stroke of the four-stroke cycle

Answer

A restricted or melted ceramic honeycomb substrate inside the catalytic converter
The correct answer identifies a restricted or melted substrate in the catalytic converter. The catalytic converter contains a dense ceramic honeycomb coated with precious metals. If engine misfires or contaminants cause overheating, the substrate can melt or crumble, restricting the exhaust stream. While small amounts of exhaust pass freely at idle, higher engine speeds produce large volumes of exhaust that back up against the blockage, creating excessive backpressure (>1.5 psi>1.5\text{ psi}) and choking engine power.

Step-by-Step Solution

1
Analyze the symptom and diagnostic test data.
The engine idles fine (low gas volume) but bogs under load (high gas volume) with elevated exhaust backpressure exceeding 1.5 psi1.5\text{ psi}.
Exhaust backpressure measurements directly evaluate physical restriction along the exhaust gas pathway.
2
Identify the component responsible for exhaust pathway restriction.
The catalytic converter's dense internal ceramic matrix can melt, shatter, or clog from unburned fuel or contaminants.
When the converter substrate is blocked, exhaust cannot exit fast enough at higher engine speeds, choking the engine.

Key Concept

Exhaust System Backpressure and Catalytic Converter Flow Restriction
Estimated Time:1m 0s
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Topics

Exhaust and Emissions Systems

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