A power balance test helps determine the source of a misfire(s). If you have misfire codes or if you have a rough-running engine, this is a good place to start your diagnosis. No matter how many cylinders you have in your engine, each cylinder is designed to produce the same amount of power. When one or more cylinders aren't producing their proper power output, you can have a misfire and also a loss of power.
One way you find the offending cylinder(s) is by doing a power balance test. While the engine is running, you cancel each cylinder one at a time and look for a drop in RPM. Each cylinder should produce the same RPM drop. If you get to a cylinder and there is no RPM drop, you've found a bad cylinder. Once you've found a bad cylinder, focus your diagnosis there to find the source of the issue.
There are a couple of different ways you can go about canceling cylinders. The easiest way is with a scan tool. Some manufacturers have built in the capacity to do a power balance test this way. You need a scanner capable of communicating with your PCM, and the vehicle needs to be capable of doing a self power balance test. If you don't have the means for that, you can use one of two methods.
- You can cancel the ignition to each cylinder one at a time. If you have a coil-on plug design, meaning you have an ignition coil for each cylinder on the engine, it's a fairly easy test. Just unplug the coils one at a time and check for the RPM drop.
- If you don't have a coil-on plug design, you can remove the spark plug boots one at a time. You need to be very careful doing this, as you can get quite a shock if you're not careful. One way to prevent getting shocked is to use a pair of plastic pliers specifically designed for this task. These are pretty handy for this sort of thing and they help protect you from the high voltage ignition circuit.
There are other ways to disable the ignition for each cylinder. Say you have a coil pack assembly that is easily accessible. You can remove each of the leads going to the coil packs and place a small piece of vacuum line in between the coil and the spark plug lead. Use just enough vacuum line to leave a 1/4-inch gap between the coil tower and ignition lead. Do this for each cylinder and start the engine. Then take a grounded test light and touch each vacuum line one by one. This shorts out the cylinders one by one and is a safer (and sometimes easier) method of disabling the ignition to each of the cylinders.
You don't have to disable the ignition to do a power balance test. If it's too difficult to get to or you're afraid of dealing with the ignition circuit, you can instead disable the fuel injectors one by one with the same result. You can simply unplug each injector one at a time and check for the same RPM drop as you did, disabling the ignition. You get virtually the same information. I often find the injectors hard to get to, so I usually go for the ignition before I look to the injectors. That might not be the case on your vehicle, and disabling the injectors might be a simpler option. Here is a video on power balance testing that will help illustrate how to perform the test and the results you're looking for.