ASE A1 Engine RepairGeneral Engine DiagnosisMedium
A customer reports that their vehicle's engine makes a 'hissing' sound that is more pronounced when accelerating and seems to reduce engine power. There are no warning lights. A quick visual inspection reveals a disconnected vacuum hose near the intake manifold. What is the most likely result of this condition?
- AReduced engine vacuum
- BA rich fuel condition
- CA lean fuel condition
- DIncreased engine power
Show answer & explanationAnswer & explanation
Correct answer: C. A lean fuel condition
A disconnected vacuum hose creates a vacuum leak, allowing unmetered air to enter the engine. This additional air is not accounted for by the mass air flow sensor, leading to a lean fuel condition.
Why the other options are wrong
- A. While a vacuum leak *causes* reduced engine vacuum, the *result* on the air/fuel mixture is a lean condition, which is the more direct consequence tested here.
- B. A vacuum leak introduces extra air, causing a lean condition, not a rich one.
- D. A vacuum leak leads to a lean condition and rough running, which *reduces* engine power, not increases it.
Vacuum Leak Effects
A vacuum leak allows unmetered air into the engine, leading to a lean air/fuel mixture, rough idle, reduced power, and sometimes a hissing sound.
- Introduces unmetered air
- Causes a lean fuel condition
- Symptoms include rough idle, reduced power, hissing noise
Memory trick: Vacuum leak, unmetered air, makes the mixture lean, I swear!