What Is Car Compression Testing? Its Process and Benefits

Any internal combustion engine needs compression to work. The operating principle remains the same in diesel as well as gasoline. To generate this compression, the fuel is mixed with the air, and it is then sparked in the area of the engine’s cylinder. The fuel-air mixture is sparked after the mixture is compressed. When the mixture is compressed, the molecules of air and fuel are very tightly packed. The gasoline engines have a spark plug, so they can efficiently operate at a moderate pressure of 140 to 160 PSI. Some engines also require a higher compression of approximately 220 PSI.

The details of the exact compression factor are listed in the user manual by the manufacturer. The crux of the matter is that the compression is necessary to operate the vehicle. A higher or lower value of compression will be an issue. A higher compression can damage the engine chambers, and the low compression will lead to loss of power. The diesel engines are slightly different. In the diesel engine, the spark plugs are absent, and the air-fuel mixture’s compression causes the fuel to ignite. In such a case, the compression value holds a lot of importance. The diesel engines usually have a minimum compression force of 350 PSI. Since the pressure is high, diesel engines are louder and heavier than petrol or gasoline engines.

What Is Car Compression Test?

There is a test to check the compression generated in the engine of the car. As the name suggests, this test is known as the car compression test. In the test, your car’s engine will be tested for the amount of generated compression it also help to know the condition of rings, valves and cylinders. Let us learn more about the engine compression test below.

How is Engine Compression Test Done?

compression tester

There are different ways to check engine compression. Some of the engines might need specialized equipment to check the compression. It is relatively simple to work with gasoline engines, but you will face significant issues with diesel engines. For the diesel engine, you will have to visit the workshop and get it tested. However, you will be able to check the compression of gasoline or petrol engine at home. Go ahead and check out the two methods to check the compression in this section.

Method I: Use Manual Compression Gauge

Manual Compression Gauge

You would need a manual compression gauge to perform this compression test. Follow the steps below to complete the test.

  • Warm up the engine and ensure that the oil is circulating properly in the engine. Trying to perform the test in a cold engine will give you significant errors.
  • Now, go ahead and disable the ignition coil. Once that is done, you would need to remove one spark plug at a time.
  • After removing the single spark plug, place the compression tester module in the hole. Allow it a few seconds so that the engine gets proper air intake. You must also hold the throttle in the fully open position.
  • Allow the engine to crank continually for about 5 to 10 revolutions to get an accurate reading. You can now record the reading.
  • Plugin the spark plug back and start the compression test with other cylinders. Record the readings.

It should be noted that a variance of 10% is acceptable, so if you notice that the compression is off by 10%, then you can consider that as a no issue. No further testing is required in such a case. If the compression is out by more than 10%, you certainly need to get the car checked by a certified mechanic.

Method II: Use Electronic Engine Analyzer

Electronic Engine Analyzer

This is the second method of checking the compression of the engine. Here, you would need to use the electronic engine analyzer. This tool shorts one cylinder at a time, and the engine keeps running. The analyzer now calculates the drop in RPM value. After all the cylinders have been shorted in a sequence, the results will be displayed. The results will show you the cylinders with high and low compression. This method is usually adopted by the workshops and technicians who have sophisticated instruments.

Understanding the Readings

  • Low Compression in All Cylinders – If too much fuel is introduced in the cylinder; the compression can fall. The fuel washes the thin layer of oil that is created to get the seal. If the car has been flooded, then you might notice this issue. The low compression in all cylinders may also indicate excessive wear and tear with the piston rings or cylinder walls. The engine would work normally, but it will not generate enough power in such a case. The second symptom of the problem is the engine throwing blue smoke. You can repeat the test by throwing a small amount of oil on the cylinder. If you notice an improvement in compression, then you know what the problem is. If there is no change in the results, then there could be an issue with the crankshaft timings and the camshaft. In such a case, you would need to check the timing belt and tune the engine again.
  • Low or 0 Compression in One Cylinder – If you are getting a 0 value or a very low value in any one cylinder, then you can be sure that the engine has damage. This could be because of the broken connection road, a broken valve spring, or even a damaged value. Bent push road and excessive camshaft wear can also cause this issue.
  • Low or 0 Compression in Two Adjacent Cylinder – This problem usually occurs when the head gasket is blown. A secondary cause of the problem is a broken camshaft in the area responsible for operating the valves of these two cylinders.
  • High Compression in Any Cylinder – High Compression is usually because of excessive carbon build-up. In this case, you will have to take out the cylinder head and clean the carbon. You may even have to opt for a chemical de-carbonization if the carbon build-up is too much.

Benefits of Compression Test

A compression test can be considered as a part of preventive maintenance. It helps you in finding out issues with the engine. For example, you will notice a loss of pressure if the piston rings have excessive wear and tear. You will also experience a loss of compression if the engine has malfunctioning valves or a blow-up gasket. On the contrary, you may even get a high compression in case of excessive carbon build-up or case of malfunctioning valves.

Once you figure out the issue, you can repair the issue. It is said that a stitch in time saves nine. The same applies to your car as well. Figuring out the problem at the start helps you prevent a major engine overhaul in the latter part of the year.


An engine Compression test can prove to be very instrumental in figuring out an issue with the engine. However, it is not necessary to do an engine test for minor problems. We would recommend you to consult the service technician. Modern cars are very complex, and they are loaded with electronic sensors. The certified service technician will be able to help you in finding out the exact problem. If you are not sure when you do the engine compression test, you can check out the other pages on our blog, and you will undoubtedly get the answer to your questions.

Leave a Comment