An engine compression test will show the state of your engine’s parts, and can potentially save you money on purchasing a replacement engine.
While today’s combustion engines are made stronger than ever, eventually the parts inside can and will wear out. As most vehicle owners know, an engine makes power by compressing vaporized fuel inside the combustion chamber. This produces a certain amount of compression (in pounds per cubic inch). When vital components including piston rings or cylinder head components wear out over time, the compression needed to efficiently burn fuel and air is reduced. If this occurs, it’s important to understand how to perform a compression test since it’s the first step to correctly diagnosing and repairing the engine.
In the information presented below, we’ll outline what a compression test is, some of the common reasons you might want to have this service completed, and how it is carried out by a professional mechanic.
What is a Compression Test?
A compression test is made to reveal the condition of your engine’s valve-train and piston rings. Particularly, parts like intake and exhaust valves, valve seats, head gaskets, and the piston rings are common components that can wear out and lead to lowered compression. While each engine and manufacturer are unique and have different levels of recommended compression psi, generally speaking, a compression over 100-psi, with less than a 10-percent variation between the lowest and highest reading is considered acceptable.
A compression test involves the use of a compression gauge, which is installed inside of the spark plug hole of each individual cylinder. As the engine is cranked over, the gauge will display the amount of compression being generated inside each cylinder.
When Would You Need a Compression Test?
Under normal circumstances, a compression test is recommended when your vehicle experiences the following symptoms:
- You observe smoke blowing from your exhaust system when you accelerate or decelerate.
- Your vehicle does not accelerate as normal or appears sluggish.
- You Observe a vibration issuing from your engine while driving down the road.
- Fuel economy is worse than normal.
- You are adding oil more often than normal.
- Your car’s engine is running hot.
How is a Compression Test Finished?
If you are thinking about finishing a compression test, there are 5 important, general steps to follow to be sure it’s as accurate as possible. Always refer to the recommended instructions for each compression tester you use to ensure accuracy.
- Warm up your engine to operating temperature.Piston rings, valve seats, and other critical parts are designed to expand as they heat, which creates the desired compression ratio inside the engine. If you complete a compression test on a cold engine, the reading will be inaccurate.
- Completely shut off the engine.To complete a compression test, the engine requires to be shut off. You should also remove the fuel pump relay switch and the electrical connection to the coil pack. This disables the ignition system and fuel delivery system, which ensures the engine does not ignite during the test.
- Disconnect spark plug wires.Be sure to disconnect them from all plugs then remove all spark plugs.
- Mount your engine compression gauge into the first spark plug hole.You’ll want to test compression on an individual cylinder basis. It’s best to start with the cylinder closest to you and move towards the back, then follow on the other side (if applicable) until you’ve finished each compression test.
- Crank the engine for short periods.Ask someone to help you by having them crank the engine over with the key many times over a period of 3 to 5 seconds. This should permit the maximum compression reading to appear on the gauge. Record this maximum number on a piece of paper per each cylinder and complete this step on each proceeding cylinder.
Once you’ve finished all the cylinders on your engine, you’ll want to review the numbers. You can refer to a service manual for your car year, make, and model to know what the numbers should look like. As we stated above, the generally accepted number is above 100-psi. The vital item to consider is the difference between each cylinder. If one is more than 10 percent less than others, a compression issue likely exists.
A compression test is mostly a good way to determine if the symptoms you’re experiencing has links with an internal engine damage. However, if compression is seen to be low inside the engine, great repairs or in some cases, complete engine replacement will be needed. The key is to have a professional mechanic carry out a compression test so they can review the results and recommend a repair or change that makes financial sense.