ASE G1 Auto Maintenance and Light RepairEngine SystemsHard
During a routine inspection, a technician notes that the engine's exhaust manifold has a visible crack. Which of the following is the most likely consequence of this condition?
- AA 'Check Engine' light due to an inaccurate oxygen sensor reading.
- BReduced engine noise and smoother operation.
- COverheating of the catalytic converter.
- DIncreased fuel economy due to reduced back pressure.
Show answer & explanationAnswer & explanation
Correct answer: A. A 'Check Engine' light due to an inaccurate oxygen sensor reading.
A cracked exhaust manifold allows unmetered air into the exhaust stream, particularly during deceleration. This 'false' air fools the upstream oxygen sensor into reading a lean condition. The Engine Control Unit (ECU) then responds by enriching the fuel mixture, which can lead to a 'Check Engine' light (often related to fuel trim or O2 sensor codes) and potentially reduced fuel economy.
Why the other options are wrong
- B. A cracked exhaust manifold typically causes increased engine noise (an exhaust leak sound), not reduced noise or smoother operation.
- C. While running rich (a potential consequence of the O2 sensor issue) can contribute to catalytic converter overheating, the direct and more immediate consequence of the crack itself is the impact on O2 sensor readings and ECU fuel control.
- D. While a crack might reduce some back pressure, the primary effect is usually on O2 sensor readings and fuel trim, not a reliable increase in fuel economy.
Cracked Exhaust Manifold Effects
A crack in the exhaust manifold can cause an exhaust leak, leading to increased noise and, critically, affecting upstream oxygen sensor readings.
- Introduces unmetered air into exhaust stream.
- O2 sensor reads false lean, causing ECU to enrich mixture.
- 'Check Engine' light common (fuel trim/O2 sensor codes).
- Increased exhaust noise and potential power loss.
Memory trick: Crack in the manifold? O2 sensor's plan is canned!