How to perform car engine compression test


When a car engine become weak and looses power due to age or any other failing reason, it is the best to go for engine compression test to mechanically diagnose the true status of engine health. A compression test is used to determine the condition of components inside the engine cylinder such as head gasket, piston rings, valves and valve seats. 

This is a very simple test and something you can do it at home even, interpreting the results is a different matter and something we will discuss later that require bit of knowledge and set rules. 

You will need a compression tester kit which can be picked up relatively cheaply from any tool shop within 50 - 100 AED (25 USD).

Engine Compression Test

  1. The first thing you need to do is to remove the fuses which supply the power to the fuel pump and injection system. You don't want fuel in the cylinders while doing this compression test. 
  2. Next remove all the spark plugs, I have covered this in another how to topic here.
  3. Then disconnect the wire supplying power to the coil to eliminate the risk of sparks or electric shock. 
  4. Screw the threaded end of the compression tester into the spark plug hole on cylinder number 1. Screw it in by hand, reasonably tight but do not use tools or over tighten. 
  5. Now hold down the throttle full and turn the engine over with the key switch. You need to turn the engine over until the reading on the tester becomes stable. I usually just turn the key and count to 5. 
  6. Take a piece of paper and write down the reading on the tester. 
  7. Now repeat the above process on cylinder number 2, then cylinder 3 etc until you have tested all cylinders in the engine. 

Petrol Engine Compression Tester.jpg    compression test.jpg  compression insert.jpg

Reading the test data

As a general rule, a healthy engine should have all cylinders reading around 100 psi or more but all engines are different and a new engine will produce a higher reading than one with a few miles on it. Anything around 130 psi or above is extremely good, anything below 90 psi is verging on extremely bad. Read about your own engine and see what the averages are. 

If you are getting very low pressure readings in any one or few of the cylinders, then perform the test again, but this time pour some oil into the cylinder through the spark plug hole before screwing the compression tester in. Not too much, around 10 millilitres is plenty. If the pressure readings are higher than before, this indicates that the piston rings are leaking. If the results of the test remain low, you should look towards a problem with the valves and seats. 

The results for all cylinders should be the same +/- a 10 psi. If one cylinder wildly varies from the rest, you know there is a problem with this cylinder and it will make further diagnosis easier because you know which cylinder to start looking for problems in. 

Another thing to look out for is that if 2 cylinders have a low pressure reading and these 2 cylinders are directly beside or opposite each other, it can indicate that the head gasket has blown between these 2 cylinders. 


**Disclaimer** I accept no responsibility for any harm or damage caused by anyone performing compression tests or misinterpreting results. Always consult a qualified mechanic and read the workshop manual for your car before performing any work. 

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Compression tests definitely have their uses, but they have drawbacks as well. For one thing, a compression test does not readily show up minor wear on rings, or on valves and valve seats because this test compresses the air in the cylinder too fast. 

As you rightly point out, the trick in diagnosing a problem lies in interpreting the results correctly, something that is very difficult for inexperienced mechanics to do accurately.

A simple compression test does not indicate the leak down rate, which is why I prefer leak down tests. With a leak down test you can actually see the leak down rate, which is a far more reliable and accurate indicator of the amount of wear in an engine. Of course, you need to be able to interpret the results correctly as well, but the advantage of a leak down test is that it will show small amounts of wear that compression tests often miss. 

On some engines these small amounts of wear cause misfiring codes, and it often happens that compression tests cannot measure these small compression losses, so in my opinion, a compression test should never be the definitive test with which to gauge engine wear.

Edited by gautam

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