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) and steady at idle. As the engine RPM is increased quickly, the vacuum momentarily drops to near zero and then recovers to a slightly higher, but still low, reading. What does this pattern most likely indicate?
- ALate ignition timing.
- BA restricted exhaust system.
- CNormal engine operation for a high-mileage engine.
- DA worn camshaft.
Show answer & explanationAnswer & explanation
Correct answer: A. Late ignition timing.
A consistently low but steady vacuum at idle, which drops even further during rapid acceleration and then recovers to a slightly higher but still low reading, is characteristic of late ignition timing. Late timing reduces cylinder filling efficiency and overall engine vacuum.
Why the other options are wrong
- B. A restricted exhaust would typically cause the vacuum to drop slowly and steadily as RPM increases, or drop from normal at idle.
- C. Normal operation would be 17-22 in. Hg, even for high-mileage engines if in good condition.
- D. A worn camshaft (specifically worn lobes) would cause a very rapid, erratic fluctuation of the vacuum gauge needle, not a steady low reading.
Vacuum Gauge - Low Steady/Drop on Accel
Interpreting vacuum gauge readings that show consistently low but steady vacuum at idle, with a significant drop on acceleration, often indicative of ignition timing issues.
- Low steady vacuum suggests general inefficiency or leaks.
- Significant drop on acceleration with recovery points to late timing.
- Late timing reduces cylinder filling and exhaust scavenging.
Memory trick: Timing's off, vacuum's cough.