Automotive Service Excellence (ASE) A8 Engine PerformanceGeneral Engine DiagnosisMedium
A vehicle owner reports a noticeable increase in fuel consumption and a slight loss of power, but no obvious misfires or smoke. A compression test reveals all cylinders are within specification (160-170 psi) and within 10% of each other. A cylinder leakage test shows less than 10% leakage on all cylinders. What is the next logical step in diagnosing the engine's condition?
- AReplace the spark plugs and ignition coils.
- BPerform a wet compression test on all cylinders.
- CCheck for a faulty fuel injector on a specific cylinder.
- DInspect the catalytic converter for restriction.
Show answer & explanationAnswer & explanation
Correct answer: D. Inspect the catalytic converter for restriction.
With good compression and leakage test results, the engine's mechanical integrity is likely sound. Increased fuel consumption and loss of power without misfires or smoke often points to a restricted exhaust system, which would hinder exhaust gas flow and reduce engine efficiency. A clogged catalytic converter is a common cause of this.
Why the other options are wrong
- A. Replacing spark plugs and coils without further diagnosis is premature, especially if there are no misfire codes or symptoms.
- B. A wet compression test is only necessary if the dry compression test reveals low compression, which is not the case here.
- C. A faulty fuel injector would likely cause a specific cylinder misfire and show up on a power balance test, which wasn't mentioned as a symptom.
Engine Condition Diagnosis - Good Compression
Diagnostic steps to take when an engine exhibits performance issues (poor fuel economy, low power) despite having good compression and minimal cylinder leakage.
- Good compression rules out major internal mechanical issues.
- Focus shifts to air/fuel, ignition, or exhaust systems.
- Restricted exhaust is a common cause of power loss and poor economy.
Memory trick: Good numbers, but still slow, where else should the diagnostics go?