ASE A1 Engine RepairGeneral Engine DiagnosisMedium
A technician is performing a cylinder balance test on a V8 engine. With the engine running, they momentarily disable Cylinder 3 and observe a significant drop in RPM. When Cylinder 7 is disabled, there is very little change in RPM. What does this indicate?
- ACylinder 3 is contributing well, and Cylinder 7 is not contributing much.
- BCylinder 3 has an open injector, and Cylinder 7 is running efficiently.
- CCylinder 3 has a weak spark, and Cylinder 7 has a good spark.
- DBoth Cylinder 3 and Cylinder 7 are misfiring.
Show answer & explanationAnswer & explanation
Correct answer: A. Cylinder 3 is contributing well, and Cylinder 7 is not contributing much.
A significant RPM drop when disabling a cylinder indicates that cylinder was actively contributing power. Little to no change in RPM indicates the disabled cylinder was already not contributing, suggesting a prior misfire or lack of combustion.
Why the other options are wrong
- B. An open injector would cause a rich condition and misfire, meaning Cylinder 3 would likely not be contributing well, resulting in little RPM change when disabled.
- C. A weak spark in Cylinder 3 would mean it's not contributing well, so disabling it would show little RPM change. A good spark in Cylinder 7 means it's contributing well, so disabling it would show a significant RPM drop.
- D. If both cylinders were misfiring, disabling either would result in little to no RPM change.
Cylinder Balance Test
A cylinder balance test evaluates the power contribution of each cylinder by momentarily disabling individual cylinders and observing the change in engine RPM or power output.
- Significant RPM drop when disabled = good cylinder.
- Little to no RPM drop when disabled = weak or dead cylinder.
- Can be performed using a scan tool or by manually disabling injectors/ignition coils.
Memory trick: Balance test: big drop, big worker; small drop, small worker.