Automotive Service Excellence (ASE) A8 Engine PerformanceGeneral Engine DiagnosisMedium
A vehicle owner reports a significant drop in fuel economy and a noticeable loss of power, but no unusual noises or visible smoke. A technician performs a cylinder power balance test. When cylinder #4 is momentarily disabled, there is a significant drop in engine RPM, similar to when any other cylinder is disabled. What does this power balance test result indicate for cylinder #4?
- ACylinder #4 has a restricted injector.
- BCylinder #4 has a severe misfire.
- CCylinder #4 is contributing power normally.
- DCylinder #4 has low compression.
Show answer & explanationAnswer & explanation
Correct answer: C. Cylinder #4 is contributing power normally.
If disabling a cylinder causes a significant drop in RPM, it means that cylinder was actively contributing power, so its removal directly impacts engine output. This indicates normal operation for that cylinder.
Why the other options are wrong
- A. A restricted injector would cause a misfire or reduced power contribution, meaning disabling it would lead to less or no RPM drop.
- B. If cylinder #4 had a severe misfire, disabling it would result in little to no change in RPM, as it wasn't contributing much power anyway.
- D. Low compression would result in reduced power contribution, so disabling the cylinder would cause a smaller RPM drop than a normally functioning cylinder.
Cylinder Power Balance Test - Normal
A diagnostic test where individual cylinders are momentarily disabled to assess their power contribution. A significant drop in engine RPM when disabled indicates normal power production.
- Performed by disabling ignition or fuel to a cylinder.
- Significant RPM drop = cylinder contributing power normally.
- Little or no RPM drop = cylinder not contributing power (misfire).
Memory trick: RPM drop is proof of power's flow.