Automotive Service Excellence (ASE) A8 Engine PerformanceFuel, Air Induction, and Exhaust Systems Diagnosis and RepairEasy
A vehicle experiences a loud 'hissing' sound from the engine compartment that increases with engine RPM. The sound is particularly noticeable during acceleration. A technician uses a smoke machine and observes smoke escaping from the intake manifold gasket area. Which of the following is the MOST likely consequence of this issue?
- AA rich air-fuel mixture.
- BA lean air-fuel mixture.
- CDecreased engine temperature.
- DExcessive exhaust back pressure.
Show answer & explanationAnswer & explanation
Correct answer: B. A lean air-fuel mixture.
A 'hissing' sound and smoke escaping from the intake manifold gasket area indicate a vacuum leak. A vacuum leak allows unmetered air to enter the engine, leading to a lean air-fuel mixture because the MAF sensor doesn't account for this extra air, and the ECM doesn't add enough fuel.
Why the other options are wrong
- A. A rich air-fuel mixture would occur if too much fuel was added or not enough air, not from a vacuum leak.
- C. A lean air-fuel mixture typically leads to increased engine temperature, not decreased.
- D. Excessive exhaust back pressure is an exhaust system issue and would not cause a hissing sound from the intake manifold.
Vacuum Leak Consequences
A vacuum leak allows unmetered air into the engine, disrupting the calculated air-fuel ratio and leading to various performance issues.
- Causes a lean air-fuel mixture.
- Can result in rough idle, misfires, and P0171/P0174 DTCs.
- Often identified by a hissing sound or smoke test.
Memory trick: Hissing intake, lean mistake, engine's ache.