ASE A1 Engine RepairGeneral Engine DiagnosisMedium
A technician is performing a power balance test on a V6 engine using a scan tool. When Cylinder 3 is momentarily disabled, the engine RPM drops significantly less than when other cylinders are disabled. What does this indicate about Cylinder 3?
- AIt is contributing less power than other cylinders.
- BIt has an over-performing fuel injector.
- CIts spark plug is fouled.
- DIt has a vacuum leak.
Show answer & explanationAnswer & explanation
Correct answer: A. It is contributing less power than other cylinders.
A smaller RPM drop when a cylinder is disabled during a power balance test indicates that the cylinder was already producing less power. If it were working normally, disabling it would cause a larger RPM drop.
Why the other options are wrong
- B. An over-performing fuel injector would cause a larger RPM drop when disabled, as it was contributing more power.
- C. A fouled spark plug would cause the cylinder to contribute less power, which would result in a smaller RPM drop when disabled.
- D. A vacuum leak would affect the entire engine or specific areas, potentially causing a misfire, but the power balance test directly measures power contribution.
Power Balance Test Interpretation
A power balance test identifies cylinders that are not contributing their fair share of power by momentarily disabling them and observing the RPM drop.
- Larger RPM drop: Cylinder was contributing good power.
- Smaller RPM drop: Cylinder was contributing less power (weak).
- No RPM drop: Cylinder was contributing no power (dead).
Memory trick: Less drop, less power, cylinder's a flop.