Automotive Service Excellence (ASE) A8 Engine PerformanceGeneral Engine DiagnosisHard
A vehicle owner reports a significant loss of power, especially noticeable during acceleration, and poor fuel economy. The engine starts and idles smoothly, and there are no abnormal noises or smoke from the exhaust. A vacuum gauge test shows a normal idle reading, but when revved, the needle drops and then slowly, sluggishly returns to the idle reading. What is the MOST likely cause?
- AA clogged fuel injector.
- BA restricted exhaust system.
- CA faulty mass airflow (MAF) sensor.
- DA severely worn camshaft.
Show answer & explanationAnswer & explanation
Correct answer: B. A restricted exhaust system.
A vacuum gauge reading that drops and then slowly returns to idle after revving, combined with loss of power and poor fuel economy, is a classic indication of a restricted exhaust system. The restriction prevents exhaust gases from exiting efficiently, leading to back pressure and reduced engine performance.
Why the other options are wrong
- A. A clogged fuel injector would likely cause a misfire, rough idle, and potentially a fluctuating vacuum reading, not the specific slow return pattern.
- C. A faulty MAF sensor can cause power loss and poor fuel economy, but would typically manifest with different vacuum gauge patterns (e.g., lean/rich conditions affecting idle vacuum) or other diagnostic trouble codes, not specifically the slow return.
- D. A severely worn camshaft would typically cause a rough idle, misfires, and a fluctuating vacuum reading, not a normal idle vacuum with a slow return after revving.
Vacuum Gauge - Restricted Exhaust
A specific vacuum gauge pattern indicating an obstruction in the exhaust system, preventing efficient gas flow.
- Normal or slightly low idle vacuum.
- Drops on acceleration.
- Slow, sluggish return to idle vacuum after revving.
- Can also cause exhaust system components to overheat.
Memory trick: Vacuum returns slow and steady, exhaust restriction is already ready!