3.3 Cylinder Power Balance and Contribution Testing

3.2 Cylinder Power Balance, Cranking, Running & Wet Compression Testing (ASE T1 Tasks A.7 & A.8)

Cylinder Power Balance Testing (ASE T1 Task A.7)

Before removing spark plugs or mechanical covers on a medium- or heavy-duty truck engine with a rough idle or misfire, a cylinder power balance test quickly identifies which cylinder (or pair of cylinders) is failing to produce its share of crankshaft torque.

Electronic vs. Manual Power Balance Methods

On modern computer-controlled truck engines, power balance testing is performed with an OEM or heavy-duty bidirectional scan tool via two primary methods:

1. Bidirectional Injector-Disable Balance Test: With the engine at normal operating temperature at curb idle and the Idle Air Control (IAC) or Electronic Throttle Control (ETC) idle-speed compensation locked by the scan tool, the technician commands each fuel injector OFF one cylinder at a time for several seconds. Disabling fuel rather than spark prevents raw unburned gasoline from entering and melting the three-way catalytic converter.
2. Crankshaft Position (CKP) Relative Contribution Test: The PCM continuously measures the time elapsed between crankshaft reluctor teeth during each cylinder's power stroke (Mode \$06 EWMA misfire counters and live cylinder contribution graphs). A firing cylinder accelerates the crankshaft; a weak or dead cylinder allows the crankshaft to decelerate.

Interpreting Power Balance RPM Drops

• Normal Equal Contribution: When each cylinder is disabled in sequence, engine speed drops by a nearly identical amount—for example, 60 to 80 RPM per cylinder on a heavy-duty V8 (or each cylinder displays near 100% relative contribution on a scan tool bar graph).
• Single Dead Cylinder (0–15 RPM drop): If disabling Cylinder 5 causes only a 5 RPM drop while the other seven cylinders each drop 70 RPM, Cylinder 5 was doing almost no work before it was disabled. Remember that a power balance test measures total cylinder output—it supports which cylinder is dead, but the root cause could be mechanical compression loss, lack of ignition spark, an open/clogged fuel injector, or a localized vacuum leak at that intake port.
• Two Adjacent Cylinders Low (5–15 RPM drop each): If two physically adjacent cylinders on the same bank (for example, Cylinders 3 and 4 on an inline-6; verify the bank and physical adjacency from the engine diagram on a V-engine) both produce minimal RPM drop, suspect a blown cylinder head gasket fire-ring land between those two bores, a shared waste-spark ignition coil pack failure, or a breached intake manifold gasket bridging the two adjacent ports.

Applied Diagnostic Check

Two weak cylinders do not automatically identify a head gasket. They may share an ignition coil or another circuit on a particular engine, and physical adjacency differs from firing-order adjacency. Verify the layout before choosing the next test. A leak-down escape into the neighboring plug hole supplies different evidence from two small RPM drops. Compare the spark and injector arrangements as well as sealing. Document which test establishes contribution and which establishes an escape path so the final conclusion does not attribute mechanical certainty to an electrical command or an RPM observation.

From a Contribution Result to a Root Cause

Consider an eight-cylinder example in which seven cut-outs cause roughly equal changes while Cylinder 5 causes almost none. First confirm the tool actually commanded the intended injector and that the engine was stable before the command. Check the cylinder numbering from the engine diagram. A scan-tool label, firing-order position and physical cylinder position are different pieces of information. A wrong mapping sends the next test to a healthy cylinder.

Next compare three fault families. Ignition diagnosis asks whether the cylinder receives the commanded primary event and a usable secondary spark under the complaint conditions. Fuel diagnosis asks whether a command produces mechanical injector flow, not merely whether a noid light flashes. Mechanical diagnosis asks whether the cylinder can fill, compress and seal. A localized intake leak or a valve that moves inadequately can reduce contribution even when a static test appears acceptable. Select the least invasive confirmatory test that distinguishes the remaining candidates.

An approved coil swap can be useful when identical coils may be exchanged. Switch the engine off, preserve connector condition and identify the original positions. If the concern moves, that is stronger component evidence than simply replacing a coil on the cylinder with the smallest RPM drop. If the concern remains, do not assume the coil test has proven the injector bad: inspect the plug, boot, wiring and mechanical measurements too.

Balance results depend on operating state. A cylinder that contributes normally at warm idle can fail under high cylinder pressure, after a hot soak or at higher speed. Conversely, an intermittent valve or injector fault may be absent during the test. Reproduce the documented load safely and use recorded misfire or contribution data when a short stationary cut-out cannot duplicate it. Do not continue a severe misfire that threatens the catalyst.

Record the baseline, each commanded cut-out, the observed change and the final verification. After repair, repeat the relevant contribution measurement and the original concern conditions. Equal contribution is a useful verification result, but complete the affected system checks and monitor requirements before returning the truck to service.