Automotive Service Excellence (ASE) A8 Engine PerformanceGeneral Engine DiagnosisEasy
A client complains that their vehicle's engine emits a constant 'hissing' sound from the intake manifold area and runs with a high, unstable idle. Which of the following is the MOST probable cause?
- AA faulty mass airflow (MAF) sensor.
- BA vacuum leak.
- CA clogged air filter.
- DA restricted exhaust system.
Show answer & explanationAnswer & explanation
Correct answer: B. A vacuum leak.
A 'hissing' sound from the intake manifold combined with a high, unstable idle are classic symptoms of a vacuum leak, where unmetered air enters the engine.
Why the other options are wrong
- A. A faulty MAF sensor could cause various drivability issues, including rough idle or stalling, but usually not a distinct 'hissing' sound from the intake manifold itself.
- C. A clogged air filter would typically cause reduced power and potentially a rich condition, not a hissing sound or high idle.
- D. A restricted exhaust system would lead to a general loss of power and potentially engine overheating, not a hissing sound from the intake or a high idle.
Vacuum Leak Symptoms
Indicators that unmetered air is entering the engine's intake system, bypassing the throttle body.
- Common symptoms include a 'hissing' sound, high/unstable idle, and lean fuel trims.
- Can be caused by cracked hoses, damaged gaskets, or loose connections.
- Diagnosed by visually inspecting, using a smoke machine, or spraying carb cleaner.
Memory trick: Hissing Intake, High Idle's Plight, Unmetered Air's the Source of Light.