How to Compression Test an Engine Out of the Car

Introduction

A compression test is a diagnostic procedure that measures the pressure inside an engine’s cylinders. This information can be used to assess the condition of the engine’s valves, piston rings, and cylinder walls. A compression test can also be used to identify potential problems, such as a blown head gasket or a cracked cylinder head.

Compression testing an engine out of the car is a relatively simple procedure that can be performed with a few basic tools. However, it is important to take some safety precautions before starting the test. These precautions include:

• Wear safety glasses and gloves.
• Disconnect the negative battery terminal.
• Allow the engine to cool down completely.

Tools and Materials

You will need the following tools and materials to perform a compression test on an engine out of the car:

• Compression tester
• Spark plug wrench
• 12-volt battery
• Jumper cables

Procedure

1. Remove the spark plugs. Use the spark plug wrench to remove all of the spark plugs from the engine. Be careful not to drop the spark plugs into the cylinders.
2. Install the compression tester. Screw the compression tester into one of the spark plug holes. Make sure that the tester is snug but do not overtighten it.
3. Connect the battery. Connect the positive terminal of the battery to the positive terminal of the compression tester. Connect the negative terminal of the battery to the negative terminal of the compression tester.
4. Crank the engine. Have an assistant crank the engine for about 5 seconds. The compression tester will measure the pressure inside the cylinder and display the reading on the gauge.
5. Record the reading. Write down the compression reading for the cylinder. Repeat steps 3-5 for each of the remaining cylinders.

Interpreting the Results

The compression readings for all of the cylinders should be within 10% of each other. If the difference between the highest and lowest reading is greater than 10%, it could indicate a problem with the engine. The following table provides a general guide to interpreting the results of a compression test:

If you get a low compression reading on one or more cylinders, it could indicate a problem with that cylinder. The most common causes of low compression include:

• Worn or damaged piston rings
• Leaking valves
• Cracked or damaged cylinder walls
• Blown head gasket

If you suspect that there is a problem with one or more of the cylinders, you should take the engine to a mechanic for further diagnosis.

Conclusion

Compression testing an engine out of the car is a simple procedure that can provide valuable information about the condition of the engine. By following the steps outlined in this article, you can safely and accurately perform a compression test on your own engine.