What Is A Compression Test – How To Do It?

Many of the car owners get confused about “what is a compression test” when they hear this term the first time. A compression test reveals what is going on with the crucial parts of your car’s engine such as the valve-train and piston rings. All these parts can wear out with time and lead to less compression. A compression test is possible with a compression gauge. This gauge gets inside of the spark plug hole and displays the amount of compression for each cylinder.

Let’s take a look at how you can perform a compression test-

What Is A Compression Test: How To Perform It Properly?

A car’s engine gets the power due to the compressed vaporized fuel that is produced into the combustion chamber. The piston rings or cylinder head components that are responsible for the compression wear out, and the compression gets compromised. This means the efficiency of fuel and air is reduced which affects the overall engine’s performance and that is why you should know.

Here are some essential steps to take while testing the engine for the compression-

To perform a compression test on a car, always warm up your engine. It will increase its temperature and prepare it for the operation. This warm-up heats the components like piston rings, valve seats, and other gears that make them expanded. A compression performed on a cold engine never gives accurate readings.

How to perform a compression test
Compression test reveals compression level of valve-train and piston ring. Source: Shutterstock


Once the warm-up is done, you need to completely shut off the engine. Also, remove the fuel pump relay switch and the electrical connection to the coil pack. This ensures that the engine won’t ignite during testing as it disables the ignition and fuel delivery system.

Next, you should get to the spark plugs and disconnect them.

It is time to install that compression gauge into the first spark plug hole. Start from the closest engine and then move to the other ones.

Something Else You Should Know About it

You may need someone’s help to crank the engine in between. Ask someone to crank it several times for 3 to 5 seconds. This will allow the compression gauge to have more accurate readings. Do this with each cylinder and note the numbers on a paper.

Generally, a compression over 100-psi having less than the 10-percent variation between the lowest and highest reading depicts an overall fine engine.

Once you have used the gauge with all the cylinders, you should have the readings on a paper. Now, you can compare these readings as stated above and get to know if there is something to worry about. Or to be clear about what does a compression test tell you, take the paper to any nearby engine repair shop.

Tip: An easy way is to check your car’s manual and determine if the readings are acceptable or you need to take your car for a repair.

A compression test is a great way to recognize your engine’s condition and how much internal damage is there. In some cases, a few repairs are performed and your engine revitalizes. A complete engine replacement can also be needed if the damage is severe; in any case, always take your car to a professional and know the exact scenario.

How to Tell If Your Car Needs a Compression Test?

 If you happen to encounter any of the following symptoms, chances are you need a compression test. Here are the symptoms:

  • When there is smoke blowing from your exhaust system. This may happen when you accelerate or decelerate.
  • When you notice a glitch every time you accelerate.
  • If your car kind of vibrates during driving.
  • When you notice a very poor fuel economy.
  • When oil doesn’t seem to be enough.
  • If the engine is running hot without any possible reason.
Compression test on a car
Warming up engine before using compressor gauge. Source: hotcars

Types of Compression Tests

There are different ways in which a compression test may take place. The most common is to apply the force to more than one axis of the material and testing it on elevated and

lowered temperatures. Uniaxial, biaxial, triaxial, cold temperature, elevated temperature, are some of the examples of different compression test methods. Experts’ Maintenance Tips always help to keep up with a car’s condition, do follow the tips and manage your car better.

The basic idea of a compression test is to apply two opposing forces that go toward each other. This force then applied to the opposite face of the sample, so the sample can be compressed. This compression is what gives the readings on the compression gauge.

What does a compression test tell you
Check car manual and find recommended compression reading. Source: Youtube


Compression testing is needed to recognise who a material reacts to compressing or squeezing. In the case of the car’s engine components, it clarifies if all the contributing parts are healthy or need a repair or replacement. You can do a compression test at home or at a professional repair point if one doesn’t want to dive into technicalities. So this was all about compression testing that we hope you know what is a compression test and relates factors.