Cylinder Power Balance Test
A cylinder power balance test is performed to locate the cylinder causing a lack of power and not contributing to the engines balance and performance. It is often used along with a cylinder compression and cylinder leakage test to pinpoint the problem.
A cylinder power balance test gives us an indication of each cylinders power, testing its effectiveness compared to the others. Each cylinder is tested at a particular speed (800-1000 RPM) and the RPM drop is recorded. The test is best performed with an engine analyzer. If an analyzer is unavailable, the technician can momentarily (a few seconds) disconnect the plug wire from its corresponding spark plug or distributor connection to disable the desired cylinder. On a distributorless system the culprit will be a coil, wire, or module. If two cylinders are dead and share a common coil, it is likely the coil is at fault. These are known as waste spark ignition systems. They use group firing.
A prolonged open in the secondary ignition system may cause damage to the coil or ignition module. Sometimes you will need to disconnect a sensor like an O2 sensor so the computer will not try to compensate for the sudden drop in RPM. Check with manufacturers specifications before removing the spark plug boot and remember safety first. Use a grabber or a tool that will protect your hand from the shock that may result from removing the plug cover.
Notice the RPM drop as the plug wire is removed from the plug. If a cylinders RPM drop is not consistent with the others, this indicates a problem with that cylinder. An engine analyzer tests each cylinder and then compares the results. An engine analyzer or dedicated tool is best used for testing COP ignition systems. This is because coil damage may result from removing a coil pack while the engine is running. Check with the manual, there are many ways to cancel a cylinder such as removing an associated connector.
The reasons for a cylinder not contributing include an ignition problem, fuel delivery, or a mechanical problem such as a faulty valve or worn piston rings. As this question suggests, there may be one or more cylinder involved. The question left out a faulty head gasket, cracked head, or block as a possible cause for low RPM drop on adjacent cylinders. This would have been a likely answer.
Remember when a question uses the word could it does not have to be the most likely or most common problem with the engine. Don't get confused. It could be leaking spark through the secondary insulation caused by poor wire insulation or wire routing. It could also be caused by clogged fuel injectors. It could also be caused by the valve train, worn piston rings, or misfires. Several things could happen to cause this condition.
