Automotive Service Excellence (ASE) A8 Engine PerformanceGeneral Engine DiagnosisMedium
A technician is diagnosing an engine that runs rough, has reduced power, and a vacuum gauge shows a steady reading of 14 in. Hg at idle. When the engine is shut off, the vacuum gauge quickly drops to zero. What does this indicate?
- AA restricted exhaust system.
- BTight valve clearances.
- CWorn valve guides.
- DA leaking intake manifold gasket.
Show answer & explanationAnswer & explanation
Correct answer: D. A leaking intake manifold gasket.
A low but steady vacuum at idle (14 in. Hg, below normal) combined with a quick drop to zero when the engine is shut off is a strong indicator of a vacuum leak, such as a leaking intake manifold gasket. A healthy engine or one with internal sealing issues (like valves) would typically hold vacuum for a period after shutdown.
Why the other options are wrong
- A. A restricted exhaust would cause a low vacuum, but it would often hold vacuum for a short time after shutdown, or possibly build up pressure.
- B. Tight valve clearances could cause low vacuum or misfires, but wouldn't necessarily cause vacuum to drop quickly to zero after shutdown, as the manifold should still be sealed.
- C. Worn valve guides would cause a fluctuating vacuum at idle and might not dissipate vacuum quickly after shutdown unless the valves were stuck open.
Vacuum Gauge - Quick Drop After Shutdown
A diagnostic observation where the vacuum gauge needle rapidly falls to zero after the engine is turned off.
- Indicates a significant vacuum leak in the intake system.
- A healthy engine should hold vacuum for a noticeable period after shutdown.
- Common culprits include intake manifold gaskets, vacuum hoses, or throttle body seals.
Memory trick: Vacuum drops fast? Intake leak won't last!