Automotive Service Excellence (ASE) A8 Engine PerformanceFuel, Air Induction, and Exhaust Systems Diagnosis and RepairMedium
A technician is diagnosing an intermittent misfire on cylinder #3. A comprehensive check reveals good spark, fuel injector pulse, and compression. The misfire is more noticeable at idle. Which of the following is the MOST likely cause?
- AA clogged catalytic converter.
- BA restricted fuel filter.
- CA faulty oxygen sensor.
- DA leaking intake manifold gasket.
Show answer & explanationAnswer & explanation
Correct answer: D. A leaking intake manifold gasket.
A leaking intake manifold gasket specifically at cylinder #3 would create a vacuum leak, allowing unmetered air into that cylinder. This would cause a lean condition and a misfire, which is often more pronounced at idle when manifold vacuum is highest. Since spark, fuel pulse, and compression are good, an air leak is the most probable cause.
Why the other options are wrong
- A. A clogged catalytic converter would cause general power loss and misfires across multiple cylinders due to backpressure, not typically an intermittent misfire on a single cylinder.
- B. A restricted fuel filter would cause a lean condition and potential misfires, but typically affecting multiple cylinders or becoming worse under load, not isolated to one cylinder and more noticeable at idle.
- C. A faulty oxygen sensor would affect fuel trims globally and could cause misfires across multiple cylinders if it leads to an excessively rich or lean condition, but not typically an intermittent misfire on a single cylinder when spark, fuel, and compression are good.
Lean Misfire (Vacuum Leak)
A vacuum leak at a specific intake manifold runner can cause a lean misfire on that cylinder, often more noticeable at idle due to higher manifold vacuum.
- Misfire isolated to one or few cylinders.
- More pronounced at idle/low RPM.
- Caused by unmetered air entry.
- Can be detected with smoke test or carb cleaner spray.
Memory trick: Misfire on one, check the air run.