Automotive Service Excellence (ASE) A8 Engine PerformanceComputerized Engine Controls Diagnosis and RepairHard
A technician is diagnosing a vehicle with a complaint of a rough idle and occasional stalling, with no stored DTCs. Using a vacuum gauge, the technician observes a steady reading of 10 in.Hg at idle, where the specification is typically 17-21 in.Hg. The needle on the vacuum gauge shows a slight, rhythmic drop and recovery. What is the MOST likely cause of this symptom?
- AA late ignition timing setting.
- BA normal condition for a high-mileage engine.
- CA restricted exhaust system.
- DA worn camshaft lobe.
Show answer & explanationAnswer & explanation
Correct answer: D. A worn camshaft lobe.
A steady but low vacuum reading (10 in.Hg instead of 17-21 in.Hg) at idle, combined with a rhythmic drop and recovery of the vacuum gauge needle, is a classic indication of a worn camshaft lobe. This causes one or more valves to open or close improperly, leading to reduced vacuum and an uneven idle.
Why the other options are wrong
- A. Late ignition timing would generally cause a lower but steady vacuum reading, without the rhythmic drop and recovery characteristic of a worn camshaft lobe.
- B. While high-mileage engines can have slightly lower vacuum, a reading of 10 in.Hg with a rhythmic fluctuation is significantly outside normal and indicates a specific problem, not just age.
- C. A restricted exhaust system would cause a normal or slightly low vacuum at idle, but as RPM increases, the vacuum would drop significantly, and the needle would be steady at idle, not rhythmic.
Vacuum Gauge Diagnosis (Worn Cam Lobe)
A vacuum gauge reading that is consistently low at idle (e.g., 10 in.Hg) but shows a slight, rhythmic drop and recovery of the needle is a strong indicator of a worn camshaft lobe, which affects valve lift and engine vacuum.
- Low vacuum indicates poor engine sealing or restricted airflow.
- Rhythmic drop suggests a consistent, cylinder-specific mechanical issue.
- Worn cam lobe affects valve timing/lift.
Memory trick: Low vacuum with a rhythmic beat? Your cam's got a worn-out feat!