What is a compression test?
Content
A compression test will show the condition of your engine parts and can potentially save you money on a new engine purchase.
While today's internal combustion engines are made stronger than ever, over time the components inside can and will wear out. As most car owners know, an engine generates power by compressing fuel vapor inside the combustion chamber. This creates a certain amount of compression (in pounds per cubic inch). When vital parts, including piston rings or cylinder head components, wear out over time, the compression ratio needed to efficiently burn fuel and air is reduced. If this happens, it's important to understand how to perform a compression test because it's the first step to properly diagnosing and repairing an engine.
In the information below, we'll cover what a compression test is, some of the common reasons you might want to have this service performed, and how a professional mechanic performs it.
What is a compression test?
The compression test is designed to check the condition of your engine's valve train and piston rings. In particular, parts such as intake and exhaust valves, valve seats, head gaskets, and piston rings are common parts that can wear and cause compression to drop. While each engine and manufacturer is unique and has different recommended compression levels, in general compression over 100 psi with less than 10 percent difference between the lowest and highest setting is considered acceptable.
Compression testing involves the use of a compression gauge that is installed inside the spark plug hole of each individual cylinder. As the engine cranks, the gauge will display the amount of compression being generated in each cylinder.
When might you need a compression check?
Under normal circumstances, a compression test is recommended if your vehicle exhibits the following symptoms:
- You notice smoke coming out of the exhaust system when you accelerate or decelerate.
- Your car does not accelerate normally or seems sluggish.
- Have you noticed a vibration coming from your engine when you are driving on the road.
- Fuel economy is worse than usual.
- You add oil more often than usual.
- Your vehicle's engine is overheated.
How is a compression test done?
If you're thinking about doing a compression test, there are 5 important general steps to follow to make sure it's as accurate as possible. Always refer to the recommended instructions for each compression tester you use to ensure accuracy.
Warm up the engine to operating temperature. Piston rings, valve seats, and other critical components are designed to expand when heated, which creates the desired compression ratio inside the engine. If you perform a compression test on a cold engine, the reading will be inaccurate.
Stop the engine completely. Stop the engine to check compression. You must also remove the fuel pump relay switch and electrical connection to the coil pack. This disables the ignition system and the fuel supply system, which ensures that the engine does not catch fire during the test.
Disconnect spark plug wires. Be sure to disconnect them from all spark plugs, then remove all spark plugs.
Install the engine compression gauge in the first hole of the spark plug. You will want to check compression in each cylinder. It's best to start with the cylinder closest to you and work towards the rear, then follow on the other side (if applicable) until you've completed each compression check.
Crank the engine for short periods of time. Have someone help you by turning the key on the engine several times within 3 to 5 seconds. At the same time, the maximum compression value should appear on the pressure gauge. Write this maximum number down on a piece of paper for each cylinder and repeat this step for each subsequent cylinder.
After you've completed all the cylinders on your engine, you'll want to look over the numbers. You can refer to the service manual for your vehicle, year, make and model to determine what the numbers should look like. As we stated above, the generally accepted value is above 100 psi. An important point to consider is the difference between each cylinder. If one of them is more than 10 percent smaller than the others, there is probably a compression problem.
A compression test is always a good way to determine if the symptoms you are experiencing are related to internal engine damage. However, if the compression in the engine is found to be low, a major overhaul or, in some cases, a complete replacement of the engine will be required. The key is to have a professional mechanic perform a compression test so they can review the results and recommend a repair or replacement that makes financial sense.
