Automotive Service Excellence (ASE) A8 Engine PerformanceGeneral Engine DiagnosisEasy
A technician is diagnosing an engine with a persistent misfire on cylinder #3. They perform a cylinder power balance test and observe a significant drop in RPM when cylinder #3's spark plug wire is momentarily disconnected, similar to other cylinders. What does this observation most likely indicate?
- ACylinder #3 has an open injector circuit.
- BCylinder #3 has a severely worn piston ring.
- CCylinder #3 is contributing power to the engine.
- DCylinder #3's spark plug is fouled.
Show answer & explanationAnswer & explanation
Correct answer: C. Cylinder #3 is contributing power to the engine.
A significant drop in RPM when a cylinder's spark is cut indicates that the cylinder was producing power before the test. If there was no drop, it would suggest the cylinder was not contributing.
Why the other options are wrong
- A. An open injector circuit would cause the cylinder to not contribute power, leading to little or no RPM drop.
- B. Severe piston ring wear would reduce compression and power, leading to less or no RPM drop when disconnected.
- D. A fouled spark plug would prevent combustion, resulting in little or no RPM drop when disconnected.
Cylinder Power Balance Test Interpretation
A diagnostic test that assesses the power contribution of individual cylinders by observing RPM changes when disabling each cylinder's ignition or fuel.
- Significant RPM drop indicates a healthy, contributing cylinder.
- Little to no RPM drop indicates a weak or dead cylinder.
- Can be performed by selectively disabling spark or fuel.
Memory trick: Balanced power makes the engine hum, imbalance tells a problem to come.