Automotive Service Excellence (ASE) A8 Engine PerformanceGeneral Engine DiagnosisEasy
A technician is diagnosing a vehicle with a consistent misfire on cylinder #2. After performing a cylinder power balance test, they notice that shorting out cylinder #2's spark plug wire causes no change in engine RPM. Shorting out other cylinders' spark plug wires causes a noticeable drop in RPM. What does this indicate?
- ACylinder #2 is not contributing power.
- BThe engine control module (ECM) is compensating for cylinder #2.
- CCylinder #2 has a rich fuel mixture.
- DCylinder #2 has an intermittent spark.
Show answer & explanationAnswer & explanation
Correct answer: A. Cylinder #2 is not contributing power.
If shorting out a cylinder's spark plug wire causes no change in engine RPM, it means that cylinder was not contributing power to begin with. The engine RPM does not drop because there's no power to lose from that cylinder.
Why the other options are wrong
- B. While the ECM might compensate, the direct observation of no RPM change points to a lack of power contribution from that cylinder.
- C. A rich mixture would still cause some power contribution, and shorting would likely show a more significant effect.
- D. An intermittent spark would still show some power contribution, and shorting would likely cause an RPM drop.
Cylinder Power Balance Test
A diagnostic test where individual cylinders are temporarily disabled to determine their power contribution to the engine's overall output.
- Disabling a good cylinder causes a significant RPM drop.
- Disabling a weak or dead cylinder causes little to no RPM drop.
- Helps identify which cylinder(s) are underperforming.
Memory trick: Power Balance: If disabling a cylinder makes NO change, it had NO power.