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Sticky Compression Test V/S Leakage Test
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Posted by wrechin2
12/6/2010 1:21 PM

Just wanted to post a test explaining the differences between the 2 test and give some examples.........

COMPRESSION TEST......What a compression test does is tells you how much pressure that piston/rings can make moving from the bottom of the cylinder to the top. The problem with a compression test is that it is not a real accurate way to determine the health of a engine as it usually has to have a very serious issue to show up on a compression test. I have a insert out of a manual that states if a engine has equal compression and is hard to start or runs poorly, to check for mechanical issues. So even OMC (Johnson/Evinrude) understands this and puts this in their service manuals. All engines work off the same principles. I have dealt with several engines that have passed a compression test and failed a leakage test.

I mainly use a compression test to help determine what octane fuel that a engine requires. The higher the compression, the more heat is produced in the combustion chamber. This can cause the temperature in the combustion chamber to ignite the fuel instead of the spark plug (detonation). Causing engine damage. The higher the octane, the slower the fuel burns at the same compression ratio. So to get the most out of the fuel, you need to use the correct octane for your compression as too high of octane will cause a 2 stroke to actually loose power. More on this later!

LEAKAGE TEST..... First you must understand what happens in a cylinder. The piston begins moving to the top of the cylinder compressing the air/fuel charge making it more flammable. Just before the piston reaches the very top of the cylinder, spark will occur. (this know as timing for example 23* Before Top Dead Center) Just as the piston reaches the top and starts on it's way back down the air/fuel that has already started to burn will finish burning creating a large amount of pressure pushing on the piston thus rotating the crankshaft, drive shaft, and then the prop. So what a leakage test does is check the seal of the piston rings and gasket. Remember when combustion is happening, there is a large amount of pressure pushing on the piston. If you have a poor seal, then some of this pressure will go elsewhere and the engine will have a power loss. This leakage is also known as blow by. Now that you understand what is going on you can see if you run 93 octane (slower burning) in a engine that needs 87 octane (faster burning) it will mess up the combustion process and cause a power loss.

What a leakage test does, is induces air into the cylinder as the engine is being held from moving and this tests the seal of the rings and head gasket. What I do is remove all the spark plugs and using a screwdriver I will rotate the piston to the top and then down 1/2" to 3/4" past the TDC (top dead center) and mark my screwdriver with tape so all are tested at the same height. I do this as a carbon ring will form at the top of the cylinder and give you a false reading. I will include Ditchdoc666's engine as a prime example of this. I will then hold the engine with either a pair of vice grips on the flywheel pushing against the starter or using a breaker bar. The leakage tester has two 100 PSI gauges. One tells me how much air I am inducing into the cylinder, I use 100 PSI as it is easier to determine %, and the other tells me how much the cylinder I am testing is holding. IF it is holding 90 PSI that is 10%, 80 PSI= 20%, 70 PSI= 30% and so on. So if the cylinder is 10% or less it is a healthy cylinder any more than that it is starting to coke or beginning to have issues. If it has 50% or 60% you have some bad issues.

I will include one that we found a coked ring on the 2nd ring. What is coking? Carbon is a by product of combustion. So coking is carbon that builds up around the rings and causes them to stick. In a automotive applications (4 strokes) the rings will rotate and will keep a lot of the carbon from sticking the rings in the ring lands. On a 2 stroke, they have pins that keep the rings from rotating. Because of this I see a lot of rings that will be stuck in the ring groove and will cause high leakage. That is why you need to regularly run chemicals like sea foam or Berryman's or anything similar to help prevent it. I will now include some examples.....

I hope this helps!!!!!!!!

Ditchdoc666's Engine. The bottom ring was so stuck I had to use pliers and break it to remove it!!!!




Posted by EricM
12/6/2010 4:18 PM
Great info James, lets us all understand a little more about our engines and how to care for them.
Posted by wrechin2
12/6/2010 4:47 PM
No problem. I have typed this many times and figure it was time to just post it.
Posted by TightlineT
12/6/2010 5:56 PM
Posted by procraft
3/6/2011 6:09 PM
what otane should a 97 jhonson 150 be using
Posted by Bubbakat
3/6/2011 6:28 PM
James if you don't mind I would like to save that and post it some where that its needed also.
Posted by wrechin2
3/6/2011 8:41 PM

procraft - 3/6/2011 5:09 PM what otane should a 97 jhonson 150 be using

Stock is 87 octane.

Posted by wrechin2
3/6/2011 8:42 PM

Bubbakat - 3/6/2011 5:28 PM James if you don't mind I would like to save that and post it some where that its needed also.

No problem.

Posted by jim_pat
3/20/2011 11:26 PM

As usual great post. In the beginning you said you mostly use the compression test for octane usage. At what compression should concider higher octane, and how do take into concideration the load factor ie, boat weight and cargo etc?
Posted by wrechin2
3/20/2011 11:56 PM
A GOOD RULE OF THUMB.....145 PSI with a bass boat type load use 93 octane with a loop charged engine. Some other factor come into play such as sleeve thickness. A 2L mercury can get away with more compression due to it having a very thick sleeve. It is the ability of heat transfer. You can also get away with more compression if you are spinning a small pitch prop. Almost like comparing walking on flat ground versus climing a mountain. On the OMC you are wanting to built, it will be very hard to get into the "high compression" range without extensive head work. That is why so many people use the 2.7L heads on 3L. I hope this helps!
Posted by slimjim
2/29/2012 9:44 PM
Thanks for this timely info. This week a local dealer was servicing my 1995 Merc 150 HP EFI and did a compression test. 3 cylinders were at 120, one at 117, one at 115, and one at 100. The engine starts first time every time and runs great. The dealer knows I would like to have a newer boat. Was he correct in telling me I'm sitting on a time bomb or was he scaring me into trading boats? Thanks for your advice.
Posted by volfans
2/29/2012 11:40 PM
what is the cost to have one of these done ????
Posted by AreTheyBiting
3/1/2012 8:15 AM
Good Post James
Thank you

Posted by wrechin2
3/2/2012 1:10 AM


As you have read there is a difference. There are a few reasons that a cylinder will have lower compression. (1) it can have scoring in the cylinder wall just like the pistons pictured, (2) the rings could have a lot of carbon on them causing them not to move freely and not seal against the cylinder wall but depending on how bad they are carboned, you can sometimes reverse it with cleaners such as seafoam but one as high as ditchdoc's a rebuild was the only way, and (3) the cylinder could have became egg shaped and this is causing the rings not to be able to seal all the way around. (4) head gasket starting to blow. Reasons 2 and 3 will just usually result in a power loss only but is recommended that you do address it. Reason 4 will cause reason 5 

Some rare reasons are (5) Bent connecting rod, (6) rod or wrist pin bearings failing, (7) modified (ground on) combustion chamber from a previously blown engine, (8) crooked deck to name a few.


 Most V-6 engines I charge $35 to do a compression and leakage test on. I highly recommend it when looking at a boat to purchase. I have rebuilt many engines that people have just purchased.


You are welcome.

Posted by slimjim
3/2/2012 9:03 AM
Thank you. As always I appreciate your advice and your willingness to share your expertise.
Posted by Squirrel Monkey
3/2/2012 8:28 PM
Very well Explained
Posted by djsmarinelectronics
11/24/2014 3:39 PM
I have been told by 3 marine dealers that you can only run a leakage test on a four stroke engine, is that true? Or did they just not have time to fool with it?

Edited by djsmarinelectronics 11/24/2014 3:54 PM
Posted by wrechin2
11/24/2014 10:36 PM
LOL!!! That is not true. I do and have done leakage test for years on 2 stroke. That would be like saying only a 4 stoke makes compression. On a leakage test the engine is held and then air is induced into the cylinder and it measures the seal of the rings and head gasket by how much is escaping (AKA blowby). On a GOOD cylinder you can measure anywhere from just past the exhaust port to TDC and it will pass by being 10% or less. Did you read Ditchdocs thread? It was all about a leakage test and how we found a stuck ring by doing one on his engine.
Posted by porthos33
11/25/2014 1:26 AM
Man you know your stuff and you tell it where it makes total sense! Appreciate the info a ton!
Posted by wrechin2
11/26/2014 6:36 PM

porthos33 - 11/25/2014 12:26 AM Man you know your stuff and you tell it where it makes total sense! Appreciate the info a ton!

you are welcome

Posted by JTM
1/30/2016 5:24 PM
The ring end gap also has a lot to do with the leak down test....
Posted by JTM
1/30/2016 5:25 PM
By the way nice post on Compression Test V/S Leakage Test....
Posted by jsb
8/10/2016 1:44 PM
A freind just got a 2002 merc efi 200hp what should the compression be on it?
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