Cylinder Power Balance Test
A cylinder power balance test tests each cylinder's effectiveness compared to the others. A weak cylinder typically results from the ignition, fuel delivery, a vacuum leak, or a mechanical problem, such as a faulty valve or worn piston rings. In addition, a faulty head gasket, cracked cylinder head, or block will cause an RPM drop on adjacent cylinders.
For many years, removing a plug boot from its plug was standard procedure. The problem is a prolonged open in the secondary ignition system may cause damage to the coil or ignition module, and intentionally dumping raw fuel into the catalytic converter is ill-advised.
Check with the manufacturer's specifications before proceeding and remember safety first. For example, sometimes a sensor, like an O2 sensor, will need to be disconnected so the computer will not try to compensate for the sudden change in conditions.
Test each cylinder at a speed of around (800-1000 RPM). Notice the drop in RPM while disabling the cylinder. If it is not consistent with the others, there is a problem with that cylinder. An engine analyzer tests each cylinder and then compares the results; if no engine analyzer is available, record the test results with a pencil and paper.
Cylinder Leak-down Test
Performed a cylinder leakage or leak-down test after a cylinder balance, and a cylinder pressure test has verified a problem with the cylinder. It shows the percentage of leakage and pinpoints the part causing the compression leak.
Listen for air hissing through the throttle body or carburetor. This sound indicates a problem with an intake valve. It allows compression to seep past the valve and its corresponding seat. Next, listen for air escaping through the tailpipe, showing a problem with a worn or burned exhaust valve. Air escaping through the oil fill cap indicates worn piston rings. Bubbles seen in the radiator neck indicate a leaking head gasket.