ASE A1 Engine RepairCylinder Head and Valve TrainHard
A technician is diagnosing an engine with a noticeable loss of power, especially at higher RPMs, and a rough idle. A vacuum gauge test shows a steady reading of 10-12 in. Hg at idle, significantly lower than the specified 17-21 in. Hg. When the engine speed is increased and then the throttle is quickly released, the needle drops to 2 in. Hg but then slowly rises to only 15 in. Hg before settling back to the low idle reading. This vacuum gauge pattern is indicative of which of the following conditions?
- ARestricted exhaust system.
- BNormal engine operation.
- CWorn piston rings.
- DLate valve timing or worn camshaft lobes.
Show answer & explanationAnswer & explanation
Correct answer: D. Late valve timing or worn camshaft lobes.
A consistently low vacuum reading at idle, coupled with a slow rise after throttle snap, is characteristic of late valve timing or worn camshaft lobes. This condition reduces the engine's ability to pull a strong vacuum due to improper valve opening/closing events.
Why the other options are wrong
- A. A restricted exhaust system would cause the vacuum to drop slowly from a high reading as RPM increases, then rise when the restriction clears, or remain low at higher RPMs. It might also show a slow return to idle vacuum after a snap, but the initial low idle vacuum points more to valve timing.
- B. Normal engine operation would show a steady high vacuum (17-21 in. Hg) and a quick rise after throttle snap.
- C. Worn piston rings typically cause a low but steady vacuum, and a rapid fluctuation of the needle at idle, not necessarily a slow rise after throttle snap.
Vacuum Gauge Diagnosis
Using a vacuum gauge to interpret engine conditions based on manifold vacuum readings and needle fluctuations under various operating states.
- Measures manifold vacuum.
- Provides insights into engine timing, compression, and breathing.
- Specific patterns indicate different engine faults.
Memory trick: Low and slow, cam or timing's woe.