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Engine compression vs leak down test expert needed
No guesses please . I need hard knowledge.
An engine's compression is typically 150 psi but two cylinders are now showing 125 psi compression (three repeat readings) but with the leak down of 10% and another cylinder a compression of 135psi with a leak down of 8 % and all the other 150 psi compression tested cylinders 5% to 10 % leak down. The question is: are good cylinder leak down test results of 10% or less plausible with same cylinder's comperession being 15 to 25 psi below typical (150 psi) for this engine? |
Is there a chance that you had any fuel or water in the cylinders with the higher readings. If it is a carb engine, did you open and close the throttle a few times? Any liquid in the cylinders will raise the compression numbers quite a bit. Enough liquid can raise it by 50 psi or more. Hope this helps, Eddie.
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Originally Posted by Young Performance
(Post 2776620)
Is there a chance that you had any fuel or water in the cylinders with the higher readings. If it is a carb engine, did you open and close the throttle a few times? Any liquid in the cylinders will raise the compression numbers quite a bit. Enough liquid can raise it by 50 psi or more. Hope this helps, Eddie.
No nothing in the cylinders for the readings mentioned. Each cylinder was repeated three or more times with minimal variation in the compression readings (while the other cylinders were also spining with the spark plugs out so anything there was blow out). The ~150 psi baseline for the engine is a good number. |
in my opinion, the leakdown numbers are the be all and end all of the diagnosis... assuming it is done properly. i have seen leak tests done at extremely low pressures and that will mask a problem but if :
1) you are saying you put 100 psi into the cylinders and it holds 90 to 92 2) you can hear where its coming from... hissing breather or exhast or intake then i would suspect you are ok... but having said that, a leak test is traditionally done at TDC. i suppose it is possible that you have bore damage below the the ring pack... like a wrist pin clip worked out and scored a bore badly... then a leak down would say everything is fine but a compression test would be low. for me, the definitive answer to your question is " if the leakdown is 10 % or less, then that is ok with me. i start looking at things seriously at 12 % or higher. |
Originally Posted by stevesxm
(Post 2776980)
in my opinion, the leakdown numbers are the be all and end all of the diagnosis... assuming it is done properly. i have seen leak tests done at extremely low pressures and that will mask a problem but if :
1) you are saying you put 100 psi into the cylinders and it holds 90 to 92 2) you can hear where its coming from... hissing breather or exhast or intake then i would suspect you are ok... but having said that, a leak test is traditionally done at TDC. i suppose it is possible that you have bore damage below the the ring pack... like a wrist pin clip worked out and scored a bore badly... then a leak down would say everything is fine but a compression test would be low. for me, the definitive answer to your question is " if the leakdown is 10 % or less, then that is ok with me. i start looking at things seriously at 12 % or higher. Bob Lloyd of Full Throttle Marine also suggested that poor ring sealing below TDC could be the cause of my lower compression but good Leak Down results. Yes to 100 psi air source but I did not track the source of the leakage ie intake / exhaust or crankcase though I definately heard leakage for all cylinders. That will be done next as well as a heavier oil in the low psi cylinders to see it that improves ring sealing. I tried a light oil (Merc Storage Seal oil) squirted into the low cylinders but that made no difference in the compression reading. Bill |
PS I have been taking the compression readings on a cold engine.
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How long has the engine been sitting without running? This also makes a difference.
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Originally Posted by QWKRN U
(Post 2777345)
How long has the engine been sitting without running? This also makes a difference.
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How many hrs on the engine?? which 2 cylinders were low?
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You got me baffled,in past experience when I find a cylinder that has 10-15 lbs less compression then the others on a motor that cranks 150 psi normally the leakdown has been much higher on those cylinders. Do you have reference compression numbers (i/e those "bad" cylinders have bumped 150 in the past? Here's something thats waay out there,what if the motor was built with some rods that were reconned and some are shorter than the others and the cylinders have always been low but you never knew it and the ring seal is fine? Smitty
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Originally Posted by HotPursuit
(Post 2777475)
How many hrs on the engine?? which 2 cylinders were low?
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Originally Posted by articfriends
(Post 2777535)
You got me baffled,in past experience when I find a cylinder that has 10-15 lbs less compression then the others on a motor that cranks 150 psi normally the leakdown has been much higher on those cylinders. Do you have reference compression numbers (i/e those "bad" cylinders have bumped 150 in the past? Here's something thats waay out there,what if the motor was built with some rods that were reconned and some are shorter than the others and the cylinders have always been low but you never knew it and the ring seal is fine? Smitty
Yes that is why I am seeking help to figure this out. So far the plausible explainations offered have been that there has been a serious deteriation in the ring sealing below TDC on the #1 and #3 cylinders (such as a wrist pin coming out and scaring the lower cylinder wall, loss of ring seal do to gas wash down) or that something in the valve train has deteriorated (cam lobe, lifter, rocker arm loosening) that reduced the intake valve open period during intake stroke. What do you think about these as potential causes of the low compression cylinders? |
i'm not trying to be a wiseass here but what i think is this...
if you are to the point now where you actually believe something is wrong, then don't try to talk yourself out of it. take it out. then take it apart to the point where you can actually confirm or deny your suspicions. if it is on the bench you can pop the rockers off, lock that cylinder 1/2 way down the bore and re do the leak down with the piston mid way . that alone will tell you a lot. you can run a borescope down there and look around. you can dial indicate the lift on the cam. there was a saying that i had taped to my tool box. it said " never check anything you don't intend to fix" i think that applies here. if something MADE you run these tests, then that intuition shouldn't be ignored. on the other hand if you just did it for something to do but really don't want to go thru the effort and expense to find out if you were right, well then just put the plugs back in and go home early. it gets to a point with all of these , what doctors call " not remarkable " diagnostic results that you simply have to make up your mind what you want to believe. either say to yourself " i believe the leak down numbers" and go drive the boat for another 50 hours and check it again. or say to yourself " i really believe the compression numbers and don't want to take the risk " and then take it apart. i really believe that there isn't any middle ground at this point. |
Originally Posted by ;2779371
i'm not trying to be a wiseass here but what i think is this...
if you are to the point now where you actually believe something is wrong, then don't try to talk yourself out of it. take it out. then take it apart to the point where you can actually confirm or deny your suspicions. if it is on the bench you can pop the rockers off, lock that cylinder 1/2 way down the bore and re do the leak down with the piston mid way . that alone will tell you a lot. you can run a borescope down there and look around. you can dial indicate the lift on the cam. there was a saying that i had taped to my tool box. it said " never check anything you don't intend to fix" i think that applies here. if something MADE you run these tests, then that intuition shouldn't be ignored. on the other hand if you just did it for something to do but really don't want to go thru the effort and expense to find out if you were right, well then just put the plugs back in and go home early. it gets to a point with all of these , what doctors call " not remarkable " diagnostic results that you simply have to make up your mind what you want to believe. either say to yourself " i believe the leak down numbers" and go drive the boat for another 50 hours and check it again. or say to yourself " i really believe the compression numbers and don't want to take the risk " and then take it apart. i really believe that there isn't any middle ground at this point. i used to have a saying taped on my tool box. it said " never check anything you don't intend to fix. " i think that applies here. if something you noticed made you run these tests then that intuition shouldn't be ignored. on the other hand if you just ran these tests for something to do then just put the plugs back in it and go home early and asssume everything will be fine. The drop in compression concerns me. If it needs to be taken appart to fix something important I will surely do that. But I just put it all back together and am not looking forward to tearing it all down again if not necessary. So I wish to be as sure as possible (with everyone's help who are willing) as to what is wrong so I can take the appropriate action to fix it if it needs fixing rather than to just tear it all totally back down only to find that it is something simple to fix like the rockers are out of adjustment. Bill |
Borescope
Stevesxm,
Where does one get ahold of a borescope suitable to look down in a engine cylinder through the spark plug hole? |
Really need to run the engine to get accurate compression results,I have checked cold engines(a couple cylinders a little low)then have ran engine and copmpression came up,with the leakdown ok this might be whats going on,snap-on makes a nice camera-scope that you can look down the cylinders,check for scoring
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running a leak down test on engines that have not run since september and its in a garage but no heat and cold here. will it be acurate or do they need to be run first (that i cant do do to being winterized) or can i just crank them over a few times then do it.
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Originally Posted by MDGperformance
(Post 2779737)
Really need to run the engine to get accurate compression results,I have checked cold engines(a couple cylinders a little low)then have ran engine and copmpression came up,with the leakdown ok this might be whats going on,snap-on makes a nice camera-scope that you can look down the cylinders,check for scoring
I will check compression on all after engine is warmed to see what the numbers are then. Thanks for the bore scope source suggestion. |
he's right. but just google it. a million people make them and ill bet there are some on e bay cheap.
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way out there
Originally Posted by articfriends
(Post 2777535)
You got me baffled,in past experience when I find a cylinder that has 10-15 lbs less compression then the others on a motor that cranks 150 psi normally the leakdown has been much higher on those cylinders. Do you have reference compression numbers (i/e those "bad" cylinders have bumped 150 in the past? Here's something thats waay out there,what if the motor was built with some rods that were reconned and some are shorter than the others and the cylinders have always been low but you never knew it and the ring seal is fine? Smitty
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Originally Posted by I'CE
(Post 2779953)
Smitty, maybe maybe not some NASCAR guys used to try and cheat by line honing a block and effectively raise the compression in an engine just a few points SOOOOOOOO yea tech.possible. ie: raising the center line of the crank in the block.
Thanks, Bill |
Originally Posted by stevesxm
(Post 2779869)
he's right. but just google it. a million people make them and ill bet there are some on e bay cheap.
Thanks, Bill |
bent rods shorten the compression distance, and you'll get lower readings, and it won't affect the leakdowns...
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Originally Posted by rssteiny
(Post 2779789)
running a leak down test on engines that have not run since september and its in a garage but no heat and cold here. will it be acurate or do they need to be run first (that i cant do do to being winterized) or can i just crank them over a few times then do it.
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it depends how you stored it. if you pulled the plugs and oiled it down and put them back and backed the rockers off so the valves were all closed and oiled behind them and sealed the exhaust and intakes and are well certain that the valve seats didnt rust up, then you could probably pull the plugs back out, re oil it hand crank it a few revolutions and leak it down and get representitive numbers. the lack of temp will leave the cleances a bit wide in the bores but, again, it will be a judgement call. if its 5 % then that tells you everything. if its 30 % that tells you everything... but you start getting to 10 or 15 % and the numbers are all the same, then whats that mean on a cold motor that you may or may not know anything about... or if its got 6 holes at 10 and 2 at 15.... ?
there is just no hard and fast answer to this question... it all has to do with how and where the motor was stored. |
Originally Posted by BUIZILLA
(Post 2780115)
bent rods shorten the compression distance, and you'll get lower readings, and it won't affect the leakdowns...
if this motor had rods bent bad enough to affect cranking compression, he wouldn't have to be wondering whether he had to take it apart. his post would have read " gee, does anyone know how to fix my back seat ? i started the boat this morning and a connecting rod went through it" |
1 Attachment(s)
Originally Posted by MDGperformance
(Post 2779737)
Really need to run the engine to get accurate compression results,I have checked cold engines(a couple cylinders a little low)then have ran engine and copmpression came up,with the leakdown ok this might be whats going on,snap-on makes a nice camera-scope that you can look down the cylinders,check for scoring
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Originally Posted by stevesxm
(Post 2780171)
if this motor had rods bent bad enough to affect cranking compression, he wouldn't have to be wondering whether he had to take it apart. his post would have read " gee, does anyone know how to fix my back seat ? i started the boat this morning and a connecting rod went through it"
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Originally Posted by Rage
(Post 2780309)
Do you know anything about the Visual Optics Video Bore Scope (picture attached) unit ie if it gives a sufficiently good visual image to diagnose cylinder wall condition?
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Originally Posted by stevesxm
(Post 2780355)
i have a milwaukee that uses the same camera technology and it can see anything. also, any of the basic non camera types on ebay ( i looked and there are a 200 of them...) will do the job as well. the only thing you have to make certain of is if it has its own light source. for a couple hundred bucks its a tool that you almost can't be without .
it is 400. My Snappy guy sold 200 of them the first month. It also has right angle attatchments and a longer probe I ordered to do Gp inspections on my Turbines. When you put the probe in ur pocket it reads the dates off quaters cristal clear. |
Originally Posted by Rage
(Post 2777015)
Thanks stevesxm,
Bob Lloyd of Full Throttle Marine also suggested that poor ring sealing below TDC could be the cause of my lower compression but good Leak Down results. Yes to 100 psi air source but I did not track the source of the leakage ie intake / exhaust or crankcase though I definately heard leakage for all cylinders. That will be done next as well as a heavier oil in the low psi cylinders to see it that improves ring sealing. I tried a light oil (Merc Storage Seal oil) squirted into the low cylinders but that made no difference in the compression reading. Bill of the leak by listening to it. Its going to be eithier the crankase, Exhaust, Or intake. I normaly pull the valve cover on the head and take the rockers off. If you find that it is comming from a valve then lightly tap the valve with a brass hammer to try to reseat it. Most of the time its a hanging guide or carbon on the exhaust seat. A cold engine will not give accurate readings on the rings the bigger the bore the worse it is. Your on to it now just keep going. Good Luck! |
Yes, with all the good information and insight everyone has provided herein and elsewhere I now know how best to proceed to sort out this issue.
I will post my findings which unfortunately will be at least a month away since that is the earliest that I will be back to the lake. Many thanks! |
Originally Posted by stevesxm
(Post 2780355)
i have a milwaukee that uses the same camera technology and it can see anything. also, any of the basic non camera types on ebay ( i looked and there are a 200 of them...) will do the job as well. the only thing you have to make certain of is if it has its own light source. for a couple hundred bucks its a tool that you almost can't be without .
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Originally Posted by stevesxm
(Post 2780171)
if this motor had rods bent bad enough to affect cranking compression, he wouldn't have to be wondering whether he had to take it apart. his post would have read " gee, does anyone know how to fix my back seat ? i started the boat this morning and a connecting rod went through it"
I've seen this too. slightly bent rod from water ingestion. NO noises, only a miss at idle, picked up the cylinder off idle. Perfect leak down, slightly low compression. |
Originally Posted by GO4BROKE
(Post 2780516)
I've seen this too. slightly bent rod from water ingestion. NO noises, only a miss at idle, picked up the cylinder off idle. Perfect leak down, slightly low compression.
so you are saying that you can bend a rod with water ingestion, such that the rings will still seal at leak down but the .002 shorter the effective rod length becomes will make a lower cranking compression number difference that you can see on an analog gage ? really ? you mean that ? and going slightly further... youre saying that you could bend a rod sufficiently thru ingestion badly enough to actually affect a cranking compression number and yet not do so much other damage like pin the rings in the lands or bend a valve or two , that you would still be scratching your head as to what else might be wrong ?.... and STILL maintain good leakdown ? well... i suppose anything is possible. but i think the realistic chances of either of those scenarios being possible is probably a 10 decimal place number away from zero. |
Originally Posted by Rage
(Post 2779185)
#1 is 125 psi, #3 is 135 psi, the rest are ~150 psi. Engine rebuilt 36 engine hours ago and all cylinders were ~150 psi after a 3 hour engine breakin. The 36 hours mostly cruising 2500 rpm and wake boarding 2200 rpms.
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Originally Posted by HotPursuit
(Post 2780693)
Not bent rods.Its side by side cylinders sounds like you hurt a head gasket.When your leaking down #1 check plug hole #3 to see if air is leaking from that plug hole.I have burnt 1 layer on a Cometic gasket or Fel Pro SS layered and had your problem.Engine ran fine.10# crossing to the next cylinder is hard to notice. Hope this helps.
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I too, noticed that the cylinders were adjacent. There's not a lot of material between cylinders. In a prior life, I blew a head gasket during a race at the Glen (BBC with aluminum heads/iron block). It torched a channel in the head between two adjacent cylinders! I used to look for relative differences in the sound level between cylinders. Typically,you'd hear almost nothing from the intakes if you had the intake off and the scope down in the port. The exhausts would be noisier measured with the headers off and the scope in the port. The big 'whoosh' would be leakage past the rings measured by dropping the probe into the crankcase. Having the intake off allows you to drope the probe down past the cam. The best stethoscope I found was one borrowed from an airline many years ago on a business trip that they now charge you $5 for to use for a few hours. Nice and flexible. Again,it's relative differences I'd look for between all the exhausts,etc. Good luck!
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Just an FYI that was mentioned in the earlier thread, http://www.offshoreonly.com/forums/g...-needed-2.html, Leak Down Test results were good for all cylinders.
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Do you have a flat tappet cam or roller. I had a flat tappet cam go down before and caused lower readings. If you have a roller it would be unlikely. I have been their before. My engine has been out 3 times. I pull my own engines and do my own assembly work. Machine work is still expensive and not having a running boat sucks. l would keep investigating and would not wait until spring when you could be boating. you can learn a lot just by pulling the intake and valve covers. Good luck!!
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