How to Do a Compression Test on Your Gas Engine

Dec 4, 2020

As an expert in the field, Grafco Electric is here to guide you through the process of performing a compression test on your gas engine. With our step-by-step instructions and valuable tips, you'll be able to ensure your engine's performance and diagnose potential issues. Let's dive into the nitty-gritty details of this essential procedure.

Understanding Compression Tests

A compression test is a diagnostic tool used to measure the compression strength of each cylinder in your gas engine. It helps identify any issues related to piston rings, valves, or cylinder head gaskets. By conducting this test, you can detect problems like low compression, leaks, or even a blown head gasket.

Step 1: Gather the Necessary Tools

Before you get started with the compression test, ensure that you have the following tools:

  • Compression tester
  • Socket set
  • Spark plug socket
  • A reliable battery or external power source
  • A partner to assist you with the test

Step 2: Prepare Your Engine

To begin the compression test, follow these steps:

  1. Ensure your engine is turned off and cool to the touch.
  2. Locate the spark plugs in your engine. They are typically attached to insulated wires and can be found on the cylinder head.
  3. Remove each spark plug, one at a time, using a spark plug socket and a ratchet or wrench.
  4. Place the spark plug wires away from the spark plug holes to prevent any accidental starting of the engine.

Step 3: Perform the Compression Test

The actual compression test can now be carried out. Follow the steps below:

  1. Connect the compression tester to one of the spark plug holes.
  2. Have your partner hold the throttle wide open or engage the engine's decompression lever (if applicable) to ensure accurate test results.
  3. Crank the engine for about 5 seconds or until the compression gauge stabilizes.
  4. Make note of the recorded compression reading for that specific cylinder.
  5. Repeat this process for each remaining cylinder in your gas engine.

Step 4: Analyze the Results

Once you've completed the compression test, it's time to interpret the results:

  • If all cylinders show similar compression readings within the manufacturer's specified range, your engine is in good condition.
  • If there's a significant difference in compression between cylinders, it may indicate problems such as worn piston rings, leaking valves, or a blown head gasket. Further diagnosis and repairs might be required.

Tips and Tricks for a Successful Compression Test

To ensure accurate results and a smooth testing process, consider the following expert tips:

  • Warm up your engine before starting the test to allow for thermal expansion.
  • Disable the fuel pump or ignition system to prevent the engine from starting accidentally while conducting the test.
  • Perform the test with a fully charged battery or connect an external power source to avoid any voltage drops affecting the test results.
  • Repeat the test if the readings differ significantly between cylinders to confirm the accuracy of your measurements.


Performing a compression test on your gas engine is a crucial step in maintaining its performance and identifying potential issues. With this detailed guide provided by Grafco Electric, you can confidently carry out the test, interpret the results, and take necessary action if required. Remember to follow safety protocols and consult a professional if you're unsure about any aspect of the test or need assistance with repairs.