Automotive Service Excellence (ASE) A8 Engine PerformanceGeneral Engine DiagnosisHard
A technician is diagnosing an engine with a complaint of significant power loss, especially under load, and a notable decrease in fuel economy. A vacuum gauge reading at idle is slightly low (15 in. Hg) and when the throttle is snapped open, the needle drops to near zero, then slowly rises to about 18 in. Hg, but then gradually drops back to 15 in. Hg and continues to slowly fall. What condition is indicated by this vacuum gauge behavior?
- ARestricted exhaust system
- BWorn piston rings
- CLate valve timing
- DLeaking head gasket
Show answer & explanationAnswer & explanation
Correct answer: A. Restricted exhaust system
A vacuum reading that drops to near zero on snap throttle, slowly rises, and then gradually falls and continues to fall indicates a restricted exhaust system. As exhaust gases build up, they impede cylinder filling, causing vacuum to drop.
Why the other options are wrong
- B. Worn piston rings usually result in a consistently low and possibly fluctuating idle vacuum, with poor response on snap throttle, but not a continuous gradual fall after a rise.
- C. Late valve timing would typically cause a consistently low vacuum at idle (e.g., 10-15 in. Hg) with a slow drop and slow rise on snap throttle, but not a continuous gradual fall after the initial rise.
- D. A leaking head gasket would likely cause low vacuum in specific cylinders, possibly with coolant in the oil, and would not typically present with this specific overall vacuum pattern.
Vacuum Gauge - Restricted Exhaust
A vacuum gauge reading pattern where idle vacuum is normal or slightly low, drops significantly on snap throttle, slowly rises, and then gradually falls back down, indicating a blockage in the exhaust system.
- Idle vacuum: Normal to slightly low (15-18 in. Hg).
- Snap throttle: Drops near zero, slowly rises to normal/above, then gradually falls.
- Caused by excessive back pressure due to blockage (e.g., clogged catalytic converter).
Memory trick: Vacuum drops then slowly fades? Exhaust is trapped in its maze.