Automotive Service Excellence (ASE) A8 Engine PerformanceComputerized Engine Controls Diagnosis and RepairEasy
A technician is performing a diagnosis on a vehicle that intermittently runs rough, especially under load, and has a P0301 DTC (Cylinder 1 Misfire Detected). The technician has already checked spark, fuel pressure, and injector pulse. The next step is to perform a cylinder power balance test. During the test, when Cylinder 1's injector is disabled, there is no change in engine RPM. When Cylinder 2's injector is disabled, the engine RPM drops significantly. What does this indicate for Cylinder 1?
- ACylinder 1 has a vacuum leak.
- BCylinder 1 has a rich mixture.
- CCylinder 1 is not contributing power.
- DCylinder 1 has a restricted exhaust valve.
Show answer & explanationAnswer & explanation
Correct answer: C. Cylinder 1 is not contributing power.
A cylinder power balance test works by temporarily disabling individual cylinders and observing the change in engine RPM. If disabling an injector (or spark) on a cylinder causes no change in RPM, it means that cylinder was not contributing power to begin with. In this case, Cylinder 1 is not contributing power, confirming it is the source of the misfire.
Why the other options are wrong
- A. A vacuum leak would cause a lean condition and a misfire, but the power balance test result (no change in RPM) specifically indicates a lack of contribution, not the root cause of the misfire.
- B. A rich mixture would still cause some combustion and power contribution, and disabling the injector would still cause an RPM drop. The symptom described is no change in RPM, indicating no contribution.
- D. A restricted exhaust valve would cause a loss of compression and power, leading to no contribution, but the balance test itself indicates the lack of contribution, not the specific cause.
Cylinder Power Balance Test Interpretation
In a cylinder power balance test, disabling a cylinder's fuel or spark and observing no change in engine RPM indicates that the cylinder was already not contributing power, confirming it as the source of a misfire.
- Disables individual cylinder's contribution.
- No RPM drop = cylinder not contributing.
- Significant RPM drop = cylinder was contributing.
Memory trick: No 'Balance' means 'No Power' from that 'Cylinder'.