Automotive Service Excellence (ASE) A8 Engine PerformanceGeneral Engine DiagnosisMedium
A technician observes a vacuum gauge reading that is steady at 21 in. Hg at idle. When the throttle is snapped open and then closed quickly, the needle drops to 2 in. Hg, then quickly rises to 28 in. Hg, before settling back to 21 in. Hg. What does this vacuum gauge reading indicate?
- ANormal engine condition.
- BA restricted exhaust system.
- CA leaking intake manifold gasket.
- DWorn valve guides.
Show answer & explanationAnswer & explanation
Correct answer: A. Normal engine condition.
A steady high vacuum (21 in. Hg) at idle, followed by a sharp drop and then an overshoot (28 in. Hg) when snapping the throttle, is a classic indication of a normally operating engine. The drop is due to sudden throttle opening, and the overshoot is due to the engine's inertia creating a strong vacuum when the throttle closes and RPMs drop.
Why the other options are wrong
- B. A restricted exhaust would typically cause a low, fluctuating vacuum at idle, and a slow return to normal after acceleration.
- C. A leaking intake manifold gasket would cause a low, steady vacuum at idle, as unmetered air reduces the vacuum level.
- D. Worn valve guides would typically cause a fluctuating vacuum at idle, often with a slight hesitation or 'float' in the needle.
Normal Vacuum Gauge Readings
Expected vacuum gauge patterns for a healthy engine under various operating conditions, indicating proper sealing and combustion.
- Steady 17-21 in. Hg at idle.
- Sharp drop on acceleration.
- Overshoot (2-5 in. Hg higher than idle) on deceleration.
- Returns to steady idle reading.
Memory trick: Idle high, snap low then soar, normal engine asks for no more!