Automotive Service Excellence (ASE) A8 Engine PerformanceGeneral Engine DiagnosisHard
A technician is diagnosing a vehicle with a complaint of significant power loss, especially noticeable during acceleration, and poor fuel economy. A vacuum gauge connected to the intake manifold shows a reading that drops to near zero during acceleration and then slowly recovers, but never reaches a steady, high reading. What is the most likely cause?
- AVacuum leak.
- BLean air-fuel mixture.
- CRestricted exhaust system.
- DLate ignition timing.
Show answer & explanationAnswer & explanation
Correct answer: C. Restricted exhaust system.
A vacuum gauge reading that drops to near zero during acceleration and slowly recovers, coupled with significant power loss and poor fuel economy, is a classic indication of a restricted exhaust system. The restriction prevents efficient exhaust gas scavenging, leading to a buildup of pressure and reduced vacuum under load.
Why the other options are wrong
- A. A vacuum leak would typically result in a consistently low and possibly fluctuating vacuum reading at idle, but not the specific behavior of dropping to near zero on acceleration and slow recovery.
- B. A lean air-fuel mixture might cause power loss and poor fuel economy, but its vacuum gauge pattern would typically be a generally low or fluctuating reading, not a near-zero drop on acceleration with slow recovery.
- D. Late ignition timing would cause lower power and poor fuel economy, and likely a lower-than-normal steady vacuum, but not the specific 'near-zero drop on acceleration with slow recovery' pattern of a restricted exhaust.
Vacuum Gauge - Restricted Exhaust
A specific vacuum gauge pattern where the reading drops significantly (often near zero) during acceleration and then slowly recovers, indicating a blockage in the exhaust system.
- Accompanied by significant power loss and poor acceleration.
- Often related to a clogged catalytic converter or collapsed muffler.
- Engine may also run hotter than normal.
Memory trick: Vacuum tells the engine's story, watch its ups and downs.