How to Use a Compression Tester | O'Reilly Auto Parts

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The compression generated by your engine is an important factor in its performance. There can be several causes of compression that’s too high or too low, and symptoms can be anything from hard starts and poor performance to misfires and even engine failure. Using a compression tester is the best way to determine if any of the symptoms you’re noticing are related to compression issues so you can work on diagnosing the underlying causes.

Tools & Materials List

Step-by-Step Instructions

Additional Resources

The compression tester is one of over eighty tools we have available in our Loaner Tool Program, and this procedure will apply to most vehicles. Our Loaner Tool program can help you save money on your repairs by allowing you to pay a deposit equal to the cost of the tool, then receive a refund of your full deposit when you return the tool to the store.

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Remove spark plug wires or ignition coils.

Step 1: Prepare Your Vehicle

Park on a level surface and open the hood. Run your vehicle’s engine until it reaches operating temperature, then shut it off. Remove your ignition coils or spark plug wires from the spark plugs.

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Loosen your spark plugs about one turn using the appropriate spark plug socket and a ratchet.

Step 2: Loosen Your Spark Plugs

Loosen your spark plugs about one turn using the appropriate spark plug socket and a ratchet.

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Use an air hose to blow out any dirt before removing the spark plugs.

Step 3: Blow Out Any Dirt or Debris

If needed, use an air hose to blow the dirt out of the spark plug walls to prevent it from falling into the combustion chamber.

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Pull the fuse to disable the fuel pump.

Step 4: Prepare to Use the Compression Tester

Remove all the spark plugs and set the throttle wide open. Pull the fuse or relay to disable the fuel pump, and determine the correct adapter from the compression tester kit.

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Get a compression reading from each cylinder.

Step 5: Get Compression Readings from Each Cylinder

Finger-tighten or hold the compression tester to a spark plug hole, then crank the engine until the highest reading is obtained, which usually takes 3 to 4 revolutions. Make note of that reading, then move the tester to each of the other cylinders until you’ve tested them all and made note of the readings.

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Reinstall your spark plugs and other components.

Step 6: Reinstall Your Components

Reinstall your spark plugs, ignition coils, and fuel pump relay or fuse. Close the hood.

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You’ll need to refer to your manufacturer’s specifications to know what compression tolerances are allowed. Most engines should have compression values over 100 PSI with less than 10 percent of variance between the highest and lowest compression measurement.

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The automotive repair work depicted in this series is performed under the direction of the manufacturer of the product featured. Prior to undertaking any of the demonstrated repairs on your vehicle, we suggest you consult with a certified mechanic or another professional who can adequately advise you of the proper repair or remedy required. O’Reilly Automotive Stores, Inc. and its affiliates (“O’Reilly”) disclaims any responsibility for injury or damage resulting from a viewer’s attempt to recreate the repairs shown in this series.

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