Automotive Service Excellence (ASE) A8 Engine PerformanceGeneral Engine DiagnosisEasy
A technician is diagnosing an engine that has a noticeable 'hissing' sound coming from the intake manifold area and a consistently high idle speed that is difficult to reduce. Which of the following is the most probable cause?
- ARestricted exhaust system
- BWorn main bearings
- CVacuum leak
- DClogged fuel injector
Show answer & explanationAnswer & explanation
Correct answer: C. Vacuum leak
A 'hissing' sound from the intake manifold area and a high, uncontrollable idle are classic symptoms of a vacuum leak, which allows unmetered air into the engine.
Why the other options are wrong
- A. A restricted exhaust system would cause a loss of power and poor acceleration, not a hissing sound or high idle.
- B. Worn main bearings would produce a dull, heavy knocking sound, not a hiss or affect idle speed in this manner.
- D. A clogged fuel injector would typically cause a misfire, rough running, and possibly a lean condition, but not a hissing sound or uncontrollably high idle.
Vacuum Leak Symptoms
Indicators that unmetered air is entering the engine's intake system, bypassing the throttle body and mass airflow sensor.
- Hissing sound from intake area.
- High, unstable, or uncontrollable idle.
- Lean fuel trims, check engine light for lean condition.
Memory trick: High idle + hiss = vacuum's kiss.