Automotive Service Excellence (ASE) A8 Engine PerformanceGeneral Engine DiagnosisMedium
A technician is performing an engine condition diagnosis on a vehicle with a complaint of a rough idle and occasional stalling. After verifying no obvious vacuum leaks or ignition issues, a vacuum gauge is connected. The gauge needle drops to virtually zero immediately after the engine is shut off. What does this indicate?
- AA significant vacuum leak.
- BA restricted exhaust system.
- CA perfectly sealed intake system.
- DNormal engine operation.
Show answer & explanationAnswer & explanation
Correct answer: A. A significant vacuum leak.
The vacuum gauge needle dropping immediately to zero after engine shut-off indicates that the intake manifold cannot hold vacuum, which is a strong sign of a significant vacuum leak in the intake system.
Why the other options are wrong
- B. A restricted exhaust system would primarily affect vacuum under load or cause a low steady vacuum, not an immediate drop to zero after shut-off.
- C. A perfectly sealed intake system would cause the vacuum gauge to hold its reading for a period after the engine is shut off, not drop to zero immediately.
- D. Normal engine operation involves the vacuum gauge holding its reading for several seconds, or slowly declining, after the engine is shut off, demonstrating the intake system's ability to hold vacuum.
Vacuum Gauge - Drop on Shut-off
A vacuum gauge needle that drops immediately to zero after the engine is turned off, indicating a failure to hold vacuum.
- Strongly suggests a significant vacuum leak.
- Confirms the intake manifold cannot maintain its vacuum.
- Requires inspection of vacuum hoses, gaskets, and components.
Memory trick: Shutdown Drop? Vacuum Leak's the Stop!