The nuts and bolts of measuring cylinder leakage

In OBD II diagnostics, most of us know that an external component like a faulty fuel injector, ignition coil, spark plug or spark plug wire is usually responsible for a P0300-series cylinder misfire trouble code. Nevertheless, even with more sophisticated onboard diagnostic strategies, the PCM usually can't pinpoint the failure that is causing the P0300 DTC. After conventional fuel, ignition and vacuum leak problems are eliminated, we're usually left with a valve train or cylinder sealing problem as a likely cause for the P0300 DTC.

Although fuel and ignition systems have dramatically changed during the past several decades, the internal components sealing cylinder compression have retained the same basic configuration. In many modern engines, cylinder-to-wall clearances have been reduced by using hypereutectic pistons and rotating friction has been reduced by using low-tension piston rings. In some cases, cylinder concentricity on assembled engines has also been improved by finish-honing the cylinder with a torque plate bolted in place. Hardened valve seats and better valve alloys have virtually eliminated burned valves. Nevertheless, a failed valve train component; a stuck, bent or burned poppet valve; a broken piston ring; or a leaking cylinder head gasket can still cause a sometimes elusive P0300 misfire problem.

Compression Testing
The traditional cranking co...