How to: Perform a Compression Test

What is a compression test?

A compression test can be a very useful tool in checking the overall health of your engine. In particular the pistons, bores, rings, valves, head gasket and even camshaft can all cause lower (or higher) than normal compression. It basically tests how well the cylinders hold compression.

What results should I be seeing?

It is widely accepted that an engine in good condition should have all cylinders within 10% of each other and have compression as close to or within the specified range for that vehicle. Any different and chances are there's a problem. I will cover this later.

How do I carry out the test?

1. First of all you need a compression tester. You can pick these up fairly cheaply (usually less than £20). I use this Gunson one avaliable from Halfrauds and a range of other stores. It's more than fine for the job.

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2. Now, you will need the engine up to working temperature to get proper readings. NOTE: From here on in be careful as some engine components may be very hot!

3. Disable the ignition and fuel supply. You can do this a number of ways but generally speaking you just need to unplug the injectors and coil-pack (etc).

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4. Next, remove the HT leads.

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5. And then the first spark plug. Then, screw the compression tester into the spark plug hole on the first cylinder. This only needs to be HAND-TIGHT. Rest the dial somewhere you can see it.

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6. Open the throttle fully by pressing the accelerator and crank the engine until the gauge stops rising. This should normally take no more than a few seconds. Note the final reading.

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7. You can now release the pressure from the cylinder and gauge by using the bleed nipple.

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8. Unscrew the compression tester and repeat for the other cylinders, recording the final reading each time.

What should I be seeing?

Readings should be within 10% or each other and within the specified range. For example:

Cylinder 1: 164psi
Cylinder 2: 170psi
Cylinder 3: 170psi
Cylinder 4: 165psi

Specified compression should be 176psi. This is a very good result and the cylinders being slightly low compression is most likely the rings and/or bores slightly worn due to mileage.

What if my results are different?

This can indicate a number of things. Next thing to do is a wet compression test (this was dry) and then go from there. This involves putting a teaspoon of oil into the cylinder just before the test. Oil will help seal the rings if they are worn and increase the reading but won't affect any suspect readings if there is an issue with the headgasket or valves.

Two cylinders next to each other with low compresson but simular readings would indicate HG failure between the two cylinders.

If one cylinder is low and the wet test increased the reading dramatically this would indicate excessive ring or bore wear. If not, you're looking at the valves, gasket or even a worn camshaft. You will need to use other methods to narrow-down the issue.
 
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As above really. You need a diesel tester because of the monster compression and you're removing the glow plugs instead of spark plugs. Apart from that the test is pretty much the same.
 
I wouldn't mind compression testing my car, it would be good to know what the readings are,

I suppose I could add that if you can't rest the gauge in a suitable place (so you can see it) then ask someone to take the reading for you
 
No just one at a time, take 1 out perform the test then put it back in then repeat

You can take out all the spark plugs at a time I think (never tried) but you are better taking one as I think you risk a light mist of fuel and oil when turning the engine over.

Just seen I've typed all the spark plugs so I will edit it, someone could have told me :lol:
 
A compression test is great for checking the over all health of an engine, pull all the plugs, disable the spark and fuel and hold the throttel open. When you find a cylinder off by about ten percent then you should do a cylinder leak down test. This will tell you if the trouble is rings/bore wear,intake or exhaust valve or headgaket/head or block crack.
 
I wouldn't mind compression testing my car, it would be good to know what the readings are,

I suppose I could add that if you can't rest the gauge in a suitable place (so you can see it) then ask someone to take the reading for you

You could but it isnt neccessary because compression guages hold the reading untill you let the pressure out with a button
 
Wrong! Wrong! Wrong!

You do it like this: Remove a spark plug,
stick your finger in the hole & see if the 'Bugger' kicks back!!:blink1::blink1:
 
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On the subject of compression. I have a 1.8t audi thats burning oil like mad, has no oil pressure & has oil seeping from the exhaust manifold.

Exhaust valve seals? Piston rings? Blocked oil strainer?

:blink:....Sorry! I seem to have strayed off subject.
 
Sounds like it has the sludge problem JC, wrong oil pretty much guarantees sludge on the 1.8T and this really does sound terminal now.
 
Sounds terminal when you start it up as well! it has a noisy valve train.

When I get round to it I'm going to try to find out what it is, also the turbo is dead. I think a wet test should tell me if its piston rings or valve seals!? Apparently the 1.8t has a common problem with the oil strainer getting blocked.

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I have another 1.8t from an older model so I should have everything I need without throwing loads of money at it.

Thanks for the advice!
 
Good post MA

A few points:

I remove all plugs so engine turns over quickly.

If you can watch the gauge this is better, as you can see whether the pressure builds up quickly to max in each bore or if one or more build up guadually, indicating a problem.

I was told many years ago to only turn engine over for approx 6 revs (same revs for each bore).

Turning the engine 6 times is enough to build max pressure in a good engine and show up any issues. If you keep turning the engine the pressure in the problem bore can build up hiding the problem.
 
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That gauge has come on in design from my early Cortina days back in the 80's loz :embarrest: The end of the pipe had a cone shaped plunger that you rammed down the spark plug hole; also remembered it coming with a remote start button kit which you connected to the battery and the starter motor for those people who had no mates to turn the ignition key for them :sad2:
 
To stop the fueling during the test just remove the EFI fuse.

Obviously put it back once the test is completed though :toung:
 
You can take out all the spark plugs at a time I think (never tried) but you are better taking one as I think you risk a light mist of fuel and oil when turning the engine over.

Just seen I've typed all the spark plugs so I will edit it, someone could have told me :lol:
I remove ALL the plugs and disconnect power to the injectors that way the starter turns over easy and no fuel vapors or smell.
If you want a more detailed report on the condition of your motor them a leakdown test is the way to go .
 
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Remember you must hold the throttle down on the floor (WOT) while cranking the motor and count the pulses so each cylinder has the same number of compression strokes for the most accurate readings.

I have always checked both hot and cold readings. Hot will / should be little higher.

IMO testing with a measured amount of oil via the centrally mounted spark plug hole on a 16 valve turbo powered engine is pointless as the oil will stay in the centre of the dished piston crown and not be able to seal around the rings to get a higher reading.

Unsure if cranking speed would be enough to throw the oil around enough so it managed to coat the rings to give a better seal for a higher reading.

Am unsure if all turbo piston crowns are dished but my 4g63 had them.

Modern turbo powered engines seem to have a higher static CR than the CR in earlier turbo powered cars so they may have flat top or even raised piston crowns so you will need to research your particular model for the type of piston used.
 
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Remember you must hold the throttle down on the floor (WOT) while cranking the motor and count the pulses so each cylinder has the same number of compression strokes for the most accurate readings.

I have always checked both hot and cold readings. Hot will / should be little higher.

IMO testing with a measured amount of oil via the centrally mounted spark plug hole on a 16 valve turbo powered engine is pointless as the oil will stay in the centre of the dished piston crown and not be able to seal around the rings to get a higher reading.

Unsure if cranking speed would be enough to throw the oil around enough so it managed to coat the rings to give a better seal for a higher reading.

Am unsure if all turbo piston crowns are dished but my 4g63 had them.

Modern turbo powered engines seem to have a higher static CR than the CR in earlier turbo powered cars so they may have flat top or even raised piston crowns so you will need to research your particular model for the type of piston used.

Can't argue with any of this :)
 
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What is a compression test?

A compression test can be a very useful tool in checking the overall health of your engine. In particular the pistons, bores, rings, valves, head gasket and even camshaft can all cause lower (or higher) than normal compression. It basically tests how well the cylinders hold compression.

What results should I be seeing?

It is widely accepted that an engine in good condition should have all cylinders within 10% of each other and have compression as close to or within the specified range for that vehicle. Any different and chances are there's a problem. I will cover this later.

How do I carry out the test?

1. First of all you need a compression tester. You can pick these up fairly cheaply (usually less than £20). I use this Gunson one avaliable from Halfrauds and a range of other stores. It's more than fine for the job.
Is it working on diesel?
 
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