Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Relative Compression Test
The relative compression test determines the relative compression per cylinder by monitoring changes in crankshaft speed during the compression stroke of each cylinder. The relative compression test is carried out by the PCM during a 10 second cranking event. Greater compression adds resistance resulting in lower crankshaft speed. The crankshaft speed data is calculated by the PCM software. The result is a difference in RPM per cylinder. This is displayed on the scan tool in bar graph form indicating each cylinder contribution.
NO RELATED
Use the Manual With the Right Hardware
Pair factory procedures with proven DIY tools so the instructions are easier to execute.
Affiliate disclosure: We may earn a commission at no extra cost to you.
When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.