How to Use a Compression Tester

Using a compression tester will help in the diagnoses of a poorly sealed cylinder on a gasoline or diesel engine. By conducting this basic test, you can tell if an engine has a severe problem by lack of compression. A lack of compression in an engine can mean a loss of power, and the engine may have to be rebuilt. There are two types of compression testers. One is a threaded screw tester that screws into the spark plug opening. The other is a hand-held tester that is rubber tipped. This type may require the help of another person to crank the engine for you as you must hold the tester in the spark plug hole during the compression test. Each tester will give close to the same results, but the threaded screw type is much more accurate in its reading.

Things You'll Need

• Specifications of cylinder pressure Socket wrench Compression tester, threaded screw or hand-held

Instructions

1. Obtain the correct specification for the cylinder's pressure reading. This pressure reading will be listed from the manufacture on the engine specifications list. There may be a range in the pressure reading that is recommend by the engine manufacture.

2. Remove the spark plug from the cylinder that you want to test by using the properly sized socket wrench.

3. Pull the center coil wire from the distributor cap on multi-cylinder engines. This will keep the engine from starting, but still allow you to "turn over" the engine, creating a pressure in the cylinders. On a pull-start type engine, such as a lawn mower or tiller, the removal of the single-cylinder spark plug will be sufficient, as the engine cannot start with the spark plug removed.

4. Install the compression tester into the spark-plug hole. If the tester is the threaded screw type, hand tightening will be sufficient, but be sure to have at least four or five rounds of threads in contact with the spark-plug hole threads. The hand-held type only needs to be pushed firmly in place with the rubber tip of the tester against the threads of the spark-plug opening.

5. Crank the engine with the starter or pull the start cord and observe the pressure gauge on the compression tester. Perform this test a few times to receive an accurate reading.

6. Compare this reading to the supplied information from the engine manufacture and follow their recommendations for repair.

7. Release the pressure from the cylinder by pressing the release valve on the side of the compression tester. Some engines may create as high as 180 lb. per square inch (PSI), and the pressure should be slowly let off of the gauge prior to its removal.

Tips & Warnings

• To create a better seal on the hand-held type compression tester, try applying a little grease around the rubber tip. The grease will aid in sealing off any gaps in the rubber to metal mating surfaces.

• Follow all engine manufacturers' process and procedures for that particular make and model. Some engines may vary in procedure for "cranking" the engine to create pressure in the cylinder.