Overview
A cylinder power balance test (also called a power balance test for short) detects cylinder(s) that are causing a lack of power. Power balance testing compares cylinder(s) that are failing to contribute to the engine’s balance and performance with normally operating cylinders (FIGURE 2-27). Ideally, all the cylinders should provide an equal amount of potential. The power balance test checks the efficiency of individual cylinders. It does not pinpoint the cause of the failure.
FIGURE 2-27 By using propane or other aerosol hydrocarbon a vacuum leak can be isolated because the engine will change the rpm when it finds the location of the failure. Be very careful with this procedure because any stray sparks or leaking spark plug wires could cause the flammable material to combust if it touches any of them.
Perform a power balance test if the vehicle has a misfire, if it performs poorly overall, or if it has failed a vacuum gauge test. A failed power balance test can be a result of a mechanical, ignition, or fuel-related fault. When used in conjunction with a mechanical compression test and cylinder leakage test, the power balance test helps to find the cause of a weak cylinder. The results from the compression and power balance tests, when combined, will pinpoint a mechanical fault.
To check each cylinder’s contribution to the engine’s total output, disable the cylinders one at a time—monitoring and recording the rpm drop. The power balance test is relative. In other words, it compares all the cylinders with each other. A noticeable rpm drop will be present on efficiently operating cylinders. A cylinder that is not contributing satisfactorily will have a reduced (or no) rpm drop compared to the properly working cylinders. The larger the difference in rpm drop, the greater the difference in each cylinder’s ability to produce its share of the power output. There is no OEM specification for the cylinder-to-cylinder balance. Any cylinder substantially different from the others is likely the cause of the performance issue.
Note: The rpm drop per cylinder during a power balance test depends on the size of the engine and its type. The fewer cylinders working, the more work each must do. Therefore, a V4 engine will have more rpm drop per cylinder than a V8 engine. If when canceling a cylinder there is little to no rpm drop, that particular cylinder is not contributing to creating engine power Skill Drill 2-7.
- Visually inspect the engine to determine the best method to disable the cylinders. If this involves connecting a scan tool, installing vacuum lines on each coil, or removing spark plug wires, prepare the engine accordingly. If necessary, disable the idle control system.
- Start the engine and allow it to idle. Record the idle rpm.
- Using the method chosen to disable cylinders, disable the first cylinder, and record the rpm. (Do not leave the cylinder disabled for more than a few seconds.)
- Reactivate the cylinder, and allow the engine to run for approximately 10 seconds to stabilize.
- Repeat the steps on each of the cylinders, and record your readings. Determine any necessary actions.
SAFETY TIPSome technicians may decide to substitute an aerosol carburetor or brake cleaner for propane. Carburetor cleaner has the same effect on a vacuum leak as propane does: a change in idle rpm or smoothness. Carefully spray the cleaner around the engine while listening for an rpm change.
What a Cylinder Balance Test Verifies
2-07 Use cylinder power balance testing to help with diagnostics.
The power balance test verifies each cylinder’s ability to create the necessary force for a well-balanced engine. The test isolates the weak cylinder(s) by comparing all cylinders with each other (relative). The power balance test, does not determine the actual cause of the fault. Power balance testing isolates the cylinders at fault. Additional testing will determine whether the fault is a compression-related, mechanical, ignition, or fuel-related problem.
Regardless of the testing procedure used, manual activation or PCM/scan tool enhanced, the results of the test are consistent and interchangeable. When performing a cylinder power balance test, the cylinders producing the least amount of change in engine rpm when disabled generate the least amount of power. Causes of weak cylinder output include a mechanical engine concern or a fault in the ignition for fuel systems. Weak cylinders will require additional testing to pinpoint the cause of low cylinder contribution.
If all the cylinders are within 5–10% of each other and the engine is operating normally, then no fault exists. Remember, though, that this is a relative test. If a fault exists that affects the entire engine, each cylinder’s output may be close to equal even though a performance problem still exists. To determine the cause, use engine vacuum testing, compression testing, and cylinder leakage testing.
TECHNICIAN TIPWhen performing the power balance test, remember that the cylinder(s) producing the least change in engine rpm when disabled are producing the least amount of power. Power balance testing does not isolate the concern; it only points the technician toward the cylinder(s) with the fault that will require additional testing.