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Published: May 11, 2012

 How to:

Perform a Compression Test...



Compression tests can often give a warning of trouble before it actually occurs.

It is a relatively simple procedure to carry out and if your car is already malfunctioning it can indicate what the fault may be.


The vehicle being used in this tutorial is a Citroen Saxo VTR



Tools you will need: Ratchet, extension for ratchet, 16mm LONG socket.

Or substitute the above for a suitable spark plug removal tool.

Compression tester

Pair of Hands




Engine Bay




Park the Vehicle ensuring handbrake is on securely and that the gear selector is in the neutral position.


Pop the Bonnet.


Make sure vehicle has cooled down before starting work so as not to harm yourself.





The above image displays the Coilpack. This device sends a High Voltage to each of the spark plugs which allows the engine to function.


High Voltages can be dangerous, for this reason disconnect the wiring plug from the distributor (circled above).



HT Leads



This image displays the HT leads. Although one is hidden hence the arrow indicating its general vicinity.


These leads carry the High Voltages from the distributor to the spark plugs.

Remove the four HT leads from the areas circled. They just pull off.


The four spark plugs will now be visible.


Remove the spark plugs using the 16mm socket/ spark plug removal tool.

If using a socket it may aid removal if you have one of these (shown below)




This allows the angle of the extension to vary and eases the removal of the plugs.


Spark Plug



Once the plugs have been removed proceed to screw the compression tester into one of the spark plug holes.


Start at one end and work your way across.


The tester should only be screwed in hand tight.


Compression tester



Note that the needle is on 0


Once the tester is inserted correctly get inside the vehicle, ensure that the gear lever is in the neutral position once more.


Hold your foot RIGHT DOWN on the throttle/ accelerator pedal.


Crank the engine over a couple of times. After a couple of revolutions the compression tester should have its maximum reading.


Take a look at the compression testerís display for that cylinder.

 Reading 1



This displays that this cylinder is producing a compression of near 13 BAR.

Anywhere in the green zone 10-14 bar is considered healthy compression.


To reset the tester press the little release button which releases the pressure. The test can then be carried out again if necessary.


Repeat the test for the other three cylinders, they should be producing similar results.


Reading 2



Reading 3



Reading 4


Generally a difference of more than two Bars between cylinders can indicate a possible fault.

The first test compression was higher than the following recordings. This could indicate a carbon build up on the piston or another possible fault.


The quicker the compression builds the healthier the engine.

If the tester shows a slow rise in pressure or jolts up progressively on each compression stroke (you may need a helper to observe the reading) this may indicate cylinder bore/ring damage or leaky valves.


Upon completion of the test Re-insert the spark plugs being careful not to over tighten them as this could strip the heads alloy thread.


Connect up the HT leads making sure they go to the same spark plugs as when they were removed.


Attach the Coilpacks electrical connector and the engine should work as it did before.


I cannot be held responsible for any damage caused whilst carrying out this test. This is merely a how to guide any mistakes made are your own.



This Article is written completely by tree. Any unauthorized copying is strictly prohibited.