Using Relative Compression to Screen Engine Mechanical Health

A vehicle comes in with a rough idle and a P0304 misfire on cylinder 4. Before pulling the spark plug and doing a manual compression test — which requires removing the coil, the plug, threading in the gauge, and cranking multiple times — perform a relative compression test with the PicoScope. It takes two minutes and tells you immediately whether the problem is mechanical.

Pull the fuel pump fuse or relay so no fuel is injected during the test. On coil-on-plug systems, you can also disconnect the coil connectors to prevent spark. You want the engine to crank freely without starting. The starter motor is your compression gauge — the current it draws on each cylinder's compression stroke is directly proportional to the compression in that cylinder.

Place the 600-amp current clamp around the negative battery cable. Open PicoScope 7 and select the Relative Compression guided test. The software sets the time base and current range automatically. Crank the engine for 8 to 10 seconds — long enough to capture multiple complete engine cycles. Stop cranking.

The waveform shows a repeating series of current peaks. Each peak represents one cylinder reaching top dead center on its compression stroke. On a healthy engine, all peaks are approximately the same height. Use the PicoScope's rotation ruler to map each peak to its corresponding cylinder number using the firing order. If cylinder 4 shows a peak that is 20 percent lower than the others — you have confirmed a mechanical issue on that cylinder. The misfire is not ignition or fuel. It is compression. Now you know to focus your testing on that cylinder — leakdown test, valve inspection, or head gasket check. If all peaks are even, compression is good across all cylinders and you can confidently move to ignition and fuel testing for the misfire cause.