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Discussion Starter · #1 · (Edited)
OK, I have been asked a few times recently if I had a how to for compression testing, so I decided to make one

A compression test will tell you if your engine has good compression. An engine is essentially a self-powered air pump, so it needs good compression to run efficiently, cleanly and to start easily.

As a rule, most engines should have 140 to 160 lbs of cranking compression with no more than 10% difference between any of the cylinders. But K series seem to be high, and diesels are higher still I think.

Low compression in one cylinder usually indicates a bad exhaust valve. Low compression in two adjacent cylinders typically means you have a bad head gasket. Low compression in all cylinders would tell you the rings and cylinders are worn and the engine needs to be overhauled. Either way an abnormal cylinder reading indicates a problem. However, these are specific examples, and in general low compression can mean all manner of stuff, one low compression does not just point to a faulty exhaust valve, the same as two does not mean head gasket, you can have a head gasket problem if the gasket has a fault next to one cylinder rather than between two.



Compression tests are best done with a warmed up engine. IF you do this then rememebr the head, spark plugs and all of the block etc will be VERY hot. Bear this in mind and either let cool slightly or wear good heat proof gloves to remove plugs

First you need to read my spark plug removal how to on my website (http://www.iain-brown.com)

So you have got as far as removing spark plugs (remove all 4) and either write on them, or attach sticky things to them as to what cylinder they came from. Of course, if your enigne is working well they should all be in the same condition and if you clean them and regap them before replaving, then this is not neccessary.

Now you can get your compression tester (can be bought from any motor factors or from ebay for about 15 pounds). K series has 14mm spark plug hole. Most testers I have seen start at 14mm and have an adapter for 16 :broon:



INsert the tester plug and tighten by hand.




Now you need to disable the fuel pump so you dont flood the cylinders with fuel. (as you have disconnected coils, then you should be safe as far as HT current goes - not sure how this goes in mems 1.9 or mems2 engines)

First locate the fuse box under the bonnet. Black plastic box to rear. Simply unclips to open



NOw locate ECU fuse. This fuse box map pic is back to front and the ECU fuses (30A) or the green ones on the right (easily seen)




Remove the 2nd fuse from the left. This allows engine to turn but fuel pump doesnt prime. If you remove first one you cant even start engine.




NOw you can do the test. Check no coils are connected, and your tester is in cylinder 1 (driver side).




Sit in driver seat and turn engine over at least 5 times,whilst keeping throttle open. Best to make sure you can see the test dial, either by arranging it so you can see from driver seat or by having a mate, and turn over engine until the dial stops rising. The engine will turn over in a pitiful sounding way. NOw go to engine bay and read the tester (or have your mate) write the values down. You can see here the reading of about 220 PSI or 15.2 Bar.



NOw press the button on the tester to release air, and remove the tester, screw itno cylinder 2 and repeat for all 4 cylinders.

If you have a discordance between cylinders of more than 10%, then you need to consider further issues (not discussed here).

This is a dry comoression test. A wet test can tell you more, if you have a problem with dry. THis involves putitng a SMALL ammount of oil in cylinder (about a teaspoon) and repeating test to compare values with dry test. I have not done this and therefore, will not discuss further here.

PLewase also note that Ex-tester fomr this forum also contributed to the information on this thread. So if you are giving rep at all, give some to him too


Hope that is useful ;)
 

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mgf
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Nice one broon - I just looked out my compression tester today in order to do this myself - but with the weather forecast for the next few days I think it may be getting left a while.
 

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Discussion Starter · #3 ·
Nice one broon - I just looked out my compression tester today in order to do this myself - but with the weather forecast for the next few days I think it may be getting left a while.
Cheers. I think you might be right :lol:
 

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I think I've just learnt something :broon:.

So as an aside, if I were to be parking in a questionable area for a few days and was to remove the 1st fuse from the front fuse box the car can't be started? just a thought to help set my mind at ease...
 

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Discussion Starter · #5 ·
I think I've just learnt something :broon:.

So as an aside, if I were to be parking in a questionable area for a few days and was to remove the 1st fuse from the front fuse box the car can't be started? just a thought to help set my mind at ease...
Absolutely. YOu turn the key and get nothing at all.
 

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other_manufacturer
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Also worth removing the pump fuse if the car hasn't been started in some time to get the oil moving around before firing up. ;)
 

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Nice one. :)

Only thing I would mention is that a K Series head gasket won't usually blow between cylinders as many others do because there is coolant in between.
 

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Discussion Starter · #9 ·
Nice one. :)

Only thing I would mention is that a K Series head gasket won't usually blow between cylinders as many others do because there is coolant in between.
Thanks for that info. My last one was indeed an external leak, rather than an "in-between" leak
 

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Pressure going downwards into the sump

I read this hoping to find the answer to my problem, the item I could not agree with that if the rings were worn and the pressure was going down then that would push the oil up the dip stick etc but this is not happening. The pressure which is going down into the sump has to go somewhere.
 

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mg_zs
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Compression values will seem ludicrously high to a lot of generic values you'll find online so only go by what's in the official books or from someone who has direct experience.

I've only ever done it on one engine and it's pretty easy to do really. Just make sure you've unplugged the big plug on the ECU loom to disconnect the fueling.
 
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