Although today’s combustion engines are manufactured stronger than ever, eventually 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 quantity of compression (in pounds per cubic inch). When vital components including piston rings or cylinder head parts wear out over time, the compression needed to efficiently burn fuel and air is lowered. If this occurs, it’s important to understand how to carry out a compression test because it’s the first step to correctly diagnosing and fixing the engine.
In the steps listed below, we’ll outline what a compression test is, some of the common reasons you might want to have this service done, 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 result to reduced 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 thought acceptable.
A compression test involves the use of a compression gauge, which is mounted inside of the spark plug hole of each individual cylinder. As the engine is cranked over, the gauge will show the amount of compression being produced inside each cylinder.
When Would You Require a Compression Test?
Under normal cases, a compression test is recommended when your vehicle experiences the following symptoms:
- You may 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 vehicle’s engine is running hot.
How is a Compression Test Completed?
If you are thinking about completing a compression test, there are 5 important, general steps to follow to ensure 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 produces the desired compression ratio inside the engine. If you finish a compression test on a cold engine, the reading will be inaccurate.
- Shut off the engine entirely.To finish a compression test, the engine has to be shut off. You should equally 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 the spark plug wires.Ensure to disconnect them from all plugs then remove all spark plugs.
- Mount your engine compression gauge into the first spark plug hole.You will want to test compression on an individual cylinder basis. It’s best to begin 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.Get someone to assist you by having them crank the engine over with the key several times over a period of 2 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 finish this step on each proceeding cylinder.
The moment you’ve completed all the cylinders on your engine, you will need to review the numbers. You should refer to a service manual for your vehicle 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 real item to consider is the difference between each cylinder. If one gets more than 10 percent less than others, a compression issue likely exists.
A compression test is often a good way to know if the symptoms you’re experiencing are related to internal engine damage. However, if compression is discovered to be low inside the engine, great repairs or in some situations, 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 advise a repair or replacement that makes financial sense.