ASE A1 Engine RepairGeneral Engine DiagnosisEasy
A technician is performing a power balance test on an engine using a scan tool. They observe that when Cylinder 2's injector is disabled, the engine RPM drops by 150 RPM. When Cylinder 4's injector is disabled, the engine RPM drops by 20 RPM. All other cylinders show an RPM drop of 140-160 RPM when disabled. What can be concluded about Cylinder 4?
- ACylinder 4 has an intermittent spark issue.
- BCylinder 4 has a strong misfire or is not firing at all.
- CCylinder 4 has a rich fuel mixture.
- DCylinder 4 is contributing more power than other cylinders.
Show answer & explanationAnswer & explanation
Correct answer: B. Cylinder 4 has a strong misfire or is not firing at all.
A minimal drop in RPM (20 RPM) when disabling Cylinder 4's injector, compared to other cylinders (140-160 RPM drop), indicates that Cylinder 4 was already contributing very little or no power, suggesting a strong misfire or complete lack of combustion.
Why the other options are wrong
- A. An intermittent spark issue would cause inconsistent misfires, but a consistently low RPM drop during the test suggests a more persistent issue with the cylinder's contribution.
- C. A rich fuel mixture would affect combustion, possibly causing a misfire or reduced power, but the direct observation of a minimal RPM drop when disabled directly points to a lack of contribution.
- D. If Cylinder 4 were contributing more power, disabling it would cause a significantly larger RPM drop than average.
Power Balance Test Interpretation
A power balance test measures the RPM drop when individual cylinders are temporarily disabled, indicating their relative power contribution to the engine.
- Large RPM drop = cylinder is contributing well.
- Small or no RPM drop = cylinder is weak or dead.
- Helps pinpoint misfiring or underperforming cylinders.
Memory trick: Small drop, small help; big drop, big help.