How a Relative Compression Test Helps Diagnose Engine Problems

At Accomplished Auto, we use a test called a relative compression test to help figure out what’s going on when your engine has a misfire or runs rough.

This test is fast, safe, and doesn’t require taking anything apart. We hook up a special tool called a lab scope and crank your engine. The tool shows a graph with lines representing each cylinder’s compression (pressure). If one line is lower than the others, that tells us one cylinder might have a problem. We also have other tools for this test as well as some vehicles. Ford for example have tools for this test in OEM capable scan tools as well.

This test doesn’t tell us exactly what’s wrong, but it gives us a strong clue. For example, if all cylinders look good, we know the problem isn’t likely caused by low compression. That means we can focus on other possible issues like spark or fuel. But if one cylinder is low, we know to check deeper for a mechanical issue, like a worn valve or piston ring.

Relative compression testing helps us save time and avoid guesswork. It’s just one of many steps we use when diagnosing engine trouble. Combined with our experienced technicians and advanced tools, we can find problems quickly and accurately—so you can get back on the road with confidence.

Need help with a rough-running engine or misfire? Bring your vehicle to Accomplished Auto in Concord, NH. We’ll find the issue and fix it right.