How to Do a Compression Test & What It Tells You


Aug 11th, 2020

How to Do a Compression Test & What It Tells You

A compression test is an excellent way to determine the condition of your engine. Among many other things, it will reveal the state of the valves, cylinders, rings, valve seats, and how these parts are wearing out.

If you notice your engine generating low power than usual or running rough, consider carrying out a compression test to find out the problem. It is also a good idea to do a compression test each time the engine gets tuned up as part of a preventative maintenance procedure. 

According to professional mechanics, a healthy engine should have compression of over 100 psi per cylinder, and the deviation between the maximum and minimum readings should not exceed 10%.

It might surprise you, but you don't need special skills to do it. You only need to learn how to interpret the results. The compression test helps you detect issues like bad valves and piston rings before they result in irreparable damage.

Below is a guide on how to do a compression test and interpret the results.

How to Prepare for a Compression Test

You need a fully charged battery when carrying out the test to get an accurate reading. You'll also need the right compression gauge for your engine. If your car runs on diesel engine, you'll require a gauge with higher compression tolerance.

1. The first step is to warm your car's engine. Remove the spark plugs one at a time before warming the engine. If a spark plug proves too hard to detach, do not remove it forcibly.

You can take your car for a quick 20-minute drive or turn it on and leave it idle. This is important as it allows the valves to sit tight and the pistons to expand and seal correctly. 

2. After the engine is all warmed up, turn off the engine and pop-up the hood.

3. Block the throttle open to ensure the engine gets plenty of air when you carry out the compression test. Disconnect the air cleaner duct or remove the air filter box from the top of the carburettor, depending on what type of fuel system you have.

4. Remove the fuel pump fuse or injection fuse if you use an electronic fuel injection system. This prevents the fuel from moving into the combustion chambers. On a diesel engine, you will have to detach the fuel shutoff solenoid.

5. Now start your car and wait until the engine stalls to ensure the remaining fuel is used. After this, you have to start working on each spark plug wire, beginning with the one closest to the front of the engine. 

6. Disconnect it first by grabbing it by the boot and twisting it to and fro as you pull it off the spark plug. In case you can't get to the boot, use spark-plug wire pliers. Once the wire is off, label it with an identification mark that shows its place on the engine. Repeat this for all the wires.

Don't forget to clean the well around each spark plug using a bicycle pump, soft brush, or compress air to prevent debris from getting into the cylinders. The presence of debris in the cylinder will scratch the wall and cause oil leaks. To clean the spark plug thoroughly, you can use compressed air or a bicycle pump and a soft brush.

7. Proceed to remove the spark plugs as it makes it easier to turn the engine when carrying out the compression test. Just as with the spark plug wires, keep track of the place of each spark plug for easier re-installation. It also helps when you want to read the spark plugs to determine combustion quality in a specific cylinder.

On a diesel engine, depending on the type, detach the glow-up plugs or the injectors into which you connect the compression gauge. If you're connecting to an injector, remember to use a heat shield for the gauge.

8. Next, kill the ignition system. Depending on your coil type, you can either disconnect the electrical connector on the coil pack or feed wire or remove the spark wire from the coil.

9. If everything is set up to this point, install the compression gauge on the first cylinder in place of the spark plug. Proceed to tighten the gauge connector to seal the cylinder to prevent compression from leaking. 

You may need to use an adapter in installing the compression gauge to the spark plug hole.

How to Do the Compression Test

You're ready to begin the test if the compression gauge is in place. 

Ask someone to start the engine or use a remote switch and watch the pressure build up on the gauge. The pressure will attain the highest reading after 4 or 5 compression strokes. Use the same number of strokes you used in the first cylinder for the rest of the cylinder, be it 4 or 6.

Note the position of the gauge needle or how far it moves on the scale. Record the compression reading on the last stroke.

Using the release valve, release the pressure on the gauge and reset to zero. After this, detach the gauge and connect it to the adjacent cylinder then repeat the test. 

When you're done, compare these results to the highest and lowest compression manufacturer specifications for your engine contained in the manual.

How to Interpret the Results of Compression Tests

If there's no problem with compression in your engine, each cylinder will evenly raise the pressure to within manufacturer specifications in the manual during the test.

Healthy petrol engines have compressions ranging anywhere between 125 and 175 psi. The difference between the cylinder with the maximum and minimum compression for a robust engine ranges from 15 to 20 psi.

Healthy diesel engines have compressions ranging from 275 to 400 psi. The variation between any two cylinders shouldn't exceed 10%.

If the results reveal compressions above the manufacturer specifications, chances are there's a buildup of carbon inside the engine cylinder. On the other hand, if they are lower than the manufacturer specifications on any cylinder, move down to the Wet Compression Test.

A compression variation over 15% between any cylinders in your results points to blown head gasket, broken valve/springs, or worn out cylinders, valve, and valve rings. If the results show low compression between two adjacent cylinders, you have a leaking head gasket.

If compression ratio is low on all cylinders, you either have a jumped timing chain/belt or worn out cylinders.

The test results usually point to a wide range of potential issues, so once you land on a problem, carry out a specific test to ascertain the results. 

What Is Wet Compression Test?  

If one (could be more than one) cylinder records a compression below the manufacturer specification, carry out the wet compression test to pinpoint the exact problem.

The test is similar to that described above except that you'll have to add a tablespoon (not more) of W30 motor oil through the spark plug hole into each cylinder you’re testing.

This test reveals either of two things, depending on the result:

  1. If the compression doesn't change, this indicates a compression leak via the head gasket and, in a few cases, a cracked cylinder head or block.
  2. If the compression increases, this indicates a compression leak via a worn-out piston ring or cylinder.

Performing a compression test is a straightforward process. No more wondering what is wrong with your engine and its component. The test helps you determine the problem and spares you from unnecessary repair costs if not identified in time.

Refer to our other articles related to compression ratio in car engines. You may also subscribe to our daily blog at to keep you updated on how to maintain your car and the latest news in the motoring world!

By Sam O.