Automotive Service Excellence (ASE) A8 Engine PerformanceGeneral Engine DiagnosisEasy
A technician is performing an engine condition diagnosis on a vehicle with a complaint of a rough idle and a slight loss of power, but no unusual noises. A vacuum gauge reading is steady at 17 in. Hg at idle. When the throttle is quickly opened and released, the needle drops to 2 in. Hg, then quickly rises to 25 in. Hg, and then settles back to 17 in. Hg. What does this vacuum gauge reading indicate?
- ARestricted exhaust system
- BNormal engine condition
- CLate valve timing
- DWorn piston rings
Show answer & explanationAnswer & explanation
Correct answer: B. Normal engine condition
A steady idle vacuum of 17 in. Hg, a quick drop to low vacuum, and a quick rise to high vacuum (25 in. Hg) before settling back to idle is indicative of a normally operating engine in good condition.
Why the other options are wrong
- A. A restricted exhaust would cause a slow drop and slow rise, often with vacuum eventually falling below idle or rising above normal at constant speed.
- C. Late valve timing would typically result in a consistently low idle vacuum and a slow, limited rise on snap throttle.
- D. Worn piston rings would cause a low and possibly fluctuating idle vacuum, and the snap throttle response would likely be sluggish or show less of a rise.
Normal Vacuum Gauge Readings
A vacuum gauge reading pattern that indicates a healthy engine: steady idle vacuum (17-22 in. Hg), quick drop on snap throttle, quick rise to high vacuum (23-25 in. Hg), then quick return to idle.
- Steady idle vacuum: 17-22 in. Hg.
- Snap throttle: Quick drop to 2-5 in. Hg.
- Snap throttle: Quick rise to 23-25 in. Hg.
- Quick return to steady idle vacuum.
Memory trick: Steady, dip, high, back: A healthy engine's vacuum track!