How to Perform a Compression Test: Step-by-Step Guide

Engine performance issues can sometimes hide in plain sight. But a compression test is one of the simplest and most effective ways to uncover internal engine problems. At ATC Cambridge, students in our auto mechanic training program get hands-on experience performing these tests under the guidance of industry-experienced instructors. 

Whether you’re dealing with misfires, loss of power, or rough idling, a compression test helps you assess the health of each cylinder quickly and accurately. Here’s a step-by-step guide to how it’s done and why it matters.

What Is a Compression Test?

A compression test measures the pressure created inside each engine cylinder during the compression stroke. This gives technicians a clear idea of how well each cylinder is sealing and compressing air/fuel mixtures, critical for engine efficiency and power.

Low or uneven compression readings often point to:

• Worn piston rings

• Leaking valves

• Blown head gaskets

• Cracked cylinder walls

These are major issues, and catching them early can save time, money, and further damage.

Step-by-Step: How to Perform a Compression Test

Step 1: Warm Up the Engine

Start the engine and let it run until it reaches normal operating temperature. Warm oil and metal parts ensure more accurate readings.

Step 2: Disable the Ignition System

To prevent the engine from starting during the test, disable the ignition system. This typically means removing the fuel pump relay and ignition coil connections.

Step 3: Remove the Spark Plugs

Carefully remove all spark plugs using a spark plug socket. Keep them in order so you can examine them for any visible signs of damage or wear.

Step 4: Insert the Compression Gauge

Thread the compression tester into the first spark plug hole. Make sure it’s secure to avoid leakage and get accurate pressure readings.

Step 5: Crank the Engine

Turn the ignition key to crank the engine for 4–6 revolutions. Watch the gauge as it climbs and stabilizes. Record the highest reading for that cylinder.

Step 6: Repeat for Each Cylinder

Remove the gauge, move to the next cylinder, and repeat the process until you’ve recorded readings for all cylinders.

Step 7: Compare the Results

Compare the pressure values for each cylinder. Ideally, all should be within 10–15% of each other. A cylinder with a significantly lower reading likely indicates internal issues that need further inspection.

What Compression Test Results Can Tell You

• Consistently low readings across all cylinders may suggest timing issues or a worn-out engine.

• A single cylinder with low compression could point to a bad valve or head gasket failure.

• Two adjacent cylinders with low compression often mean a blown head gasket between them.

At an auto mechanic school like ATC Cambridge, students learn not only how to run the test but also how to interpret the results and recommend the correct next steps.

Why Compression Testing Matters in Auto Mechanic Training

Compression testing bridges the gap between diagnostics and repairs. It teaches auto mechanic training students how to:

• Evaluate engine performance

• Communicate findings clearly to customers

• Make repair decisions backed by data

• Save time and avoid unnecessary part replacements

This is why compression testing is a core skill covered in every automotive training module. ATC Cambridge students graduate with the confidence to tackle advanced diagnostics, right from their first day in the field.

Are you looking for comprehensive automotive training?

Contact ATC Cambridge for more information.