Automotive Service Excellence (ASE) A8 Engine PerformanceGeneral Engine DiagnosisMedium
A technician observes a vacuum gauge reading that is consistently low (around 10-12 in. Hg) while the engine is idling. When the throttle is quickly opened and then closed, the needle drops slightly, then slowly rises to only about 15 in. Hg before returning to the low idle reading. What is the most probable cause of this vacuum gauge behavior?
- AWorn piston rings
- BVacuum leak
- CRestricted exhaust system
- DLate valve timing
Show answer & explanationAnswer & explanation
Correct answer: D. Late valve timing
A consistently low vacuum at idle (10-12 in. Hg) combined with a slow drop and slow, limited rise on snap throttle is characteristic of late valve timing, often due to a jumped timing chain or belt.
Why the other options are wrong
- A. Worn piston rings would typically cause a low and possibly fluctuating idle vacuum but would often show a quicker, though still low, response on snap throttle.
- B. A vacuum leak usually causes a high or fluctuating idle, and while it results in low vacuum, the snap throttle response isn't typically characterized by a slow, limited rise.
- C. A restricted exhaust would typically cause the vacuum to rise above normal on deceleration or gradually fall after a snap throttle, not a consistently low idle vacuum with a slow recovery.
Vacuum Gauge - Late Valve Timing
A vacuum gauge reading indicating consistently low vacuum at idle (10-15 in. Hg), with a slow drop on snap throttle and a slow, limited rise before returning to the low idle, suggesting valve timing is retarded.
- Consistently low idle vacuum (10-15 in. Hg).
- Snap throttle: Slow drop, slow and limited rise.
- Caused by camshaft or crankshaft timing misalignment.
Memory trick: Low vacuum, slow moves? Timing's off, the engine grooves.