ASE A1 Engine RepairGeneral Engine DiagnosisMedium
A vehicle owner reports a noticeable decrease in engine power and fuel economy, accompanied by a 'whooshing' or 'hissing' sound that is more prominent under acceleration. A visual inspection reveals a torn vacuum hose connected to the intake manifold. What is the MOST direct effect of this torn hose on engine operation?
- AOver-enrichment of the air-fuel mixture
- BLean air-fuel mixture due to unmetered air
- CReduced fuel pressure to the injectors
- DExcessive exhaust back pressure
Show answer & explanationAnswer & explanation
Correct answer: B. Lean air-fuel mixture due to unmetered air
A torn vacuum hose connected to the intake manifold allows unmetered air to enter the engine. This additional air bypasses the mass air flow (MAF) sensor, leading the engine control unit (ECU) to calculate an incorrect (too low) fuel delivery based on metered air. The result is a lean air-fuel mixture, causing a lack of power and reduced fuel economy.
Why the other options are wrong
- A. Unmetered air causes a lean condition, not an over-enrichment (rich condition).
- C. A vacuum leak typically doesn't directly reduce fuel pressure; rather, it affects the air side of the air-fuel ratio.
- D. Exhaust back pressure issues are unrelated to a vacuum leak and typically manifest differently (e.g., restricted exhaust flow).
Vacuum Leak Symptoms
Signs that unmetered air is entering the engine's intake system, disrupting the air-fuel ratio.
- Symptoms: rough idle, stalling, lean condition, check engine light (DTCs for lean mixture).
- Often accompanied by a hissing or whistling sound.
- Can be detected by spraying carburetor cleaner or smoke testing around vacuum lines.
Memory trick: Vacuum's hiss, a lean mix, something's amiss.