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Power Balance Testing How-To

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One of the most basic and useful diagnostic tools in a good mechanic's arsenal is power balance testing. A PBT can find several potential problems, but it's most often used as a means of finding a misfire, fuel/spark/air problem, or problems specific to a cylinder.

On late model vehicles, you can often plug in a scan tool and monitor cylinder output through a scan tool. When this isn't possible, a mechanical PBT should be done. Use caution when conducting this test as large amounts of power are involved in the ignition system of an automobile.

The procedure is to remove spark from each cylinder in succession, testing the output of the remainder. Any variance will pinpoint the cylinder of concern. Using either the spark plug connector (either its individual coil, if equipped, or wire) or the coil pack that provides power to those plugs (does a job similar to a distributor on an older vehicle).

With the engine running and proper safety in place, disconnect spark from each cylinder, one at a time, and watch for engine output changes. The RPM should drop by the same amount for each cylinder as it's disconnected and go back to normal when it's reconnected. Any variance points to a faulty cylinder.

When doing this, you'll be disconnecting and reconnecting each cylinder one at a time. Do not disconnect the cylinder for too long - a second or two should give you what you need. When a cylinder isn't firing, raw fuel is being passed through into the exhaust, which can cause problems later.

Once you've found the bad cylinder, a compression or leak-down test will find any mechanical issues in the cylinder itself. Other likely issues that could be at play include vacuum leaks, spark delivery (plug, coil), fuel delivery, etc. A PBT is a first-step diagnosis that helps you narrow down the issue to find the cause.

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