Leakdown test Results ?
07-16-2010, 05:44 AM
#17
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When I get that motor together with new rings, gaskets and valve job I'll come back to this thread and post a comparison table of original 240.000Km and freshly rebuilt 'just run in' readings.
07-16-2010, 06:20 AM
#18
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Thanks Dave
I've never used a leakdown tester, always just used a Compression tester, and your tests with a compression tester and oil were sufficient to illustrate the issue as a ring problem. so I will continue to lay my faith in the humble compression test results.
Thanks Cheers Roy
I've never used a leakdown tester, always just used a Compression tester, and your tests with a compression tester and oil were sufficient to illustrate the issue as a ring problem. so I will continue to lay my faith in the humble compression test results.
Thanks Cheers Roy
07-16-2010, 07:05 AM
#19
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Hi Roy
I'll also continue to lay my faith in compression tests ... but leakdown/pressure testing can give a bit more information regarding the degree of wear, once you have a good cylinder to compare to, or using a traditional percentage gauge as a baseline of known good/average/bad results. Pressurising a cylinder also lets you know where any leakage is going ... so it has its place as a further test to confirm/give more info about what you might suspect from a compression test.
I'll also continue to lay my faith in compression tests ... but leakdown/pressure testing can give a bit more information regarding the degree of wear, once you have a good cylinder to compare to, or using a traditional percentage gauge as a baseline of known good/average/bad results. Pressurising a cylinder also lets you know where any leakage is going ... so it has its place as a further test to confirm/give more info about what you might suspect from a compression test.
07-16-2010, 07:58 AM
#20
Racer
Thread Starter
Hi Dave,
My compression test figures are below.
Looking at those It seems clear that something is up with cylinder 5 - would you agree?
Given that there is very little change to the figures after adding oil that would seem to suggest that either, the problem is in the valves rather than rings, or that the ring damage is so severe that adding oil has made no difference.
The leakdown test for that cylinder (in fact all of them) confirmed that there was no leakage past either of the valves so that would seem to indicate very poor rings. The strange thing is that the leakdown figures for that cylinder were no worse than the other three on that bank, where I would have expected very poor figures. That is what has me confused at the moment.
My compression test figures are below.
Looking at those It seems clear that something is up with cylinder 5 - would you agree?
Given that there is very little change to the figures after adding oil that would seem to suggest that either, the problem is in the valves rather than rings, or that the ring damage is so severe that adding oil has made no difference.
The leakdown test for that cylinder (in fact all of them) confirmed that there was no leakage past either of the valves so that would seem to indicate very poor rings. The strange thing is that the leakdown figures for that cylinder were no worse than the other three on that bank, where I would have expected very poor figures. That is what has me confused at the moment.
Last edited by heliflyer; 03-20-2012 at 07:20 AM.
07-16-2010, 11:33 AM
#21
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Jay
#1,2,3&4 are good compression figures dry (180psi trend average) and then with oil they rise about twenty pounds ... so all seems good for bank 1 -4.
#5,6&7 are lower dry compression figures by around twenty pounds with #8 lower by 10psi ... with a wet rise of 5, 30,40 and 20 respectively. The noticeable things with this bank are a trend of dry lower compressions, abnormally high wet rise on 7 of 40psi, and abnormally low wet rise on 5 of 5psi.
Something is up on the 5-8 bank. The lower dry compressions across the board and the lack of wet response on 5 could be cross leakage on the head gasket ??? (although I would have thought that would show as water jacket pressurisation/leakage)... but that's about the only thing left if you've eliminated valves. The abnormally high wet response on 7 however points to rings because it raised it to a normal reading just like mine. Maybe there are several issues on that bank?? It's weird that a new head gasket on 5-8 did nothing to improve dry compressions. The only other grasp at straw guess that I have is a warped head.
I think you need to do a double check of dry,wet, and holding pressure tests on bank 5-8 to be sure there isn't a stray reading.
#1,2,3&4 are good compression figures dry (180psi trend average) and then with oil they rise about twenty pounds ... so all seems good for bank 1 -4.
#5,6&7 are lower dry compression figures by around twenty pounds with #8 lower by 10psi ... with a wet rise of 5, 30,40 and 20 respectively. The noticeable things with this bank are a trend of dry lower compressions, abnormally high wet rise on 7 of 40psi, and abnormally low wet rise on 5 of 5psi.
Something is up on the 5-8 bank. The lower dry compressions across the board and the lack of wet response on 5 could be cross leakage on the head gasket ??? (although I would have thought that would show as water jacket pressurisation/leakage)... but that's about the only thing left if you've eliminated valves. The abnormally high wet response on 7 however points to rings because it raised it to a normal reading just like mine. Maybe there are several issues on that bank?? It's weird that a new head gasket on 5-8 did nothing to improve dry compressions. The only other grasp at straw guess that I have is a warped head.
I think you need to do a double check of dry,wet, and holding pressure tests on bank 5-8 to be sure there isn't a stray reading.
Last edited by Dave928S; 07-16-2010 at 11:53 AM.
07-16-2010, 12:48 PM
#23
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Jay
#1,2,3&4 are good compression figures dry (180psi trend average) and then with oil they rise about twenty pounds ... so all seems good for bank 1 -4.
#5,6&7 are lower dry compression figures by around twenty pounds with #8 lower by 10psi ... with a wet rise of 5, 30,40 and 20 respectively. The noticeable things with this bank are a trend of dry lower compressions, abnormally high wet rise on 7 of 40psi, and abnormally low wet rise on 5 of 5psi.
Something is up on the 5-8 bank. The lower dry compressions across the board and the lack of wet response on 5 could be cross leakage on the head gasket ??? (although I would have thought that would show as water jacket pressurisation/leakage)... but that's about the only thing left if you've eliminated valves. The abnormally high wet response on 7 however points to rings because it raised it to a normal reading just like mine. Maybe there are several issues on that bank?? It's weird that a new head gasket on 5-8 did nothing to improve dry compressions. The only other grasp at straw guess that I have is a warped head.
I think you need to do a double check of dry,wet, and holding pressure tests on bank 5-8 to be sure there isn't a stray reading.
#1,2,3&4 are good compression figures dry (180psi trend average) and then with oil they rise about twenty pounds ... so all seems good for bank 1 -4.
#5,6&7 are lower dry compression figures by around twenty pounds with #8 lower by 10psi ... with a wet rise of 5, 30,40 and 20 respectively. The noticeable things with this bank are a trend of dry lower compressions, abnormally high wet rise on 7 of 40psi, and abnormally low wet rise on 5 of 5psi.
Something is up on the 5-8 bank. The lower dry compressions across the board and the lack of wet response on 5 could be cross leakage on the head gasket ??? (although I would have thought that would show as water jacket pressurisation/leakage)... but that's about the only thing left if you've eliminated valves. The abnormally high wet response on 7 however points to rings because it raised it to a normal reading just like mine. Maybe there are several issues on that bank?? It's weird that a new head gasket on 5-8 did nothing to improve dry compressions. The only other grasp at straw guess that I have is a warped head.
I think you need to do a double check of dry,wet, and holding pressure tests on bank 5-8 to be sure there isn't a stray reading.
07-16-2010, 01:02 PM
#24
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A compression tester is ok for a quick check, and can point out some valve timing issues, but there are way to many variables, IE cranking speed, worn or out of time cams, throttle position, number of compression strokes for it to be of much use other than, "We have a problem at one or more cylinders".
There is a reason that we use leakdown testers when lives are on the line. (aircraft recips)
How to use one is in AC 43- 18 (I think), it should be on line some place.
Like it or not, a uniform leakdown tester, not something cobbled together, is THE standard for measuring combustion chamber health.
A compression tester is just another tool in the box that pro's reach for to reach a sound diagnostic answer to a problem, along with many other tools.
About the only time I use one is when I want to make sure there is enough squeeze in a cylinder to make one work.
To the op, with the limited about of relative information we have here (with out knowing how many times the compression gauge saw compression and throttle position and cranking speed) my guess is a bad ring or the head gasket is not sealing.
A wet test is designed to take the rings out of the picture for the most part, and yours only came up slightly.
How much oil did you put in the cylinder? what kind? what about the other things I mentioned?
There is a reason that we use leakdown testers when lives are on the line. (aircraft recips)
How to use one is in AC 43- 18 (I think), it should be on line some place.
Like it or not, a uniform leakdown tester, not something cobbled together, is THE standard for measuring combustion chamber health.
A compression tester is just another tool in the box that pro's reach for to reach a sound diagnostic answer to a problem, along with many other tools.
About the only time I use one is when I want to make sure there is enough squeeze in a cylinder to make one work.
To the op, with the limited about of relative information we have here (with out knowing how many times the compression gauge saw compression and throttle position and cranking speed) my guess is a bad ring or the head gasket is not sealing.
A wet test is designed to take the rings out of the picture for the most part, and yours only came up slightly.
How much oil did you put in the cylinder? what kind? what about the other things I mentioned?
07-16-2010, 01:14 PM
#25
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Another data point to add to Greg's excellent analysis about leakdown tests. An engine will usually make rated power even when the leakdown shows significant leakage. Even more than 15%. The reason is that the sealing of the rings happens a bit different during engine operation than during a leakdown test. Teledyne Continental once built an engine with NO engine rings, and tested it. It made rated power for a few minutes until the plugs oil fouled.
And I agree with Bob, investigate the cam on the upper bank. It appears to be mistimed or has other issues. If the heads were shaved at any time, this could also account for the aggregation of lower comp on that bank.
And I agree with Bob, investigate the cam on the upper bank. It appears to be mistimed or has other issues. If the heads were shaved at any time, this could also account for the aggregation of lower comp on that bank.
07-16-2010, 01:20 PM
#26
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Valve mis-timing doesn't allow cylinder to develop full pressure on compression stroke but valves are still closed at TDC which gives good leakdown result.
Just double checking the obvious ... was throttle jammed full open during compression testing???
07-16-2010, 01:29 PM
#27
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Another data point to add to Greg's excellent analysis about leakdown tests. An engine will usually make rated power even when the leakdown shows significant leakage. Even more than 15%. The reason is that the sealing of the rings happens a bit different during engine operation than during a leakdown test. Teledyne Continental once built an engine with NO engine rings, and tested it. It made rated power for a few minutes until the plugs oil fouled.
And I agree with Bob, investigate the cam on the upper bank. It appears to be mistimed or has other issues. If the heads were shaved at any time, this could also account for the aggregation of lower comp on that bank.
And I agree with Bob, investigate the cam on the upper bank. It appears to be mistimed or has other issues. If the heads were shaved at any time, this could also account for the aggregation of lower comp on that bank.
07-16-2010, 01:41 PM
#28
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I've checked a number of 928s with my leakdown tester (Tavia - made in USA) and I have always gotten 5% or less on a warm motor (and even a cold one recently), including my car at 200K miles, except for one car that had a leaky head gaskets (inadequately torqued Cometic) and was found on disassembly to have stone-honed cylinder walls. That car had a bunch of cylinders at 12% (and 150 or so compression test) right after the so-called "rebuild" and over time a couple of them went to 28%.
07-16-2010, 01:58 PM
#29
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I've checked a number of 928s with my leakdown tester (Tavia - made in USA) and I have always gotten 5% or less on a warm motor (and even a cold one recently), including my car at 200K miles, except for one car that had a leaky head gaskets (inadequately torqued Cometic) and was found on disassembly to have stone-honed cylinder walls. That car had a bunch of cylinders at 12% (and 150 or so compression test) right after the so-called "rebuild" and over time a couple of them went to 28%.
But the oil consumption seems to prove that, many of these motors seem to get better (less) oil consumption with age.
07-16-2010, 07:49 PM
#30
Racer
Thread Starter
Thanks guys for all the comments and suggestions
I will double check the timing in the high bank.
The compression tests were done with the throttle closed but each cylinder cranked for 7 secs.
Hopefully warped heads are not an issue as both heads were decked when I had them off to renew the head gaskets. I Also asked the machine shop to ensure they removed the same amount from each head to avoid any water bridge problems.
I'm away for a few days but will report back later with more results prior to pulling the engine. I'm going to pull the engine regardless because I'm not confident of getting a good job done on the cam carriers with the engine in the car.
Again thanks for your input and discussion.
I will double check the timing in the high bank.
The compression tests were done with the throttle closed but each cylinder cranked for 7 secs.
Hopefully warped heads are not an issue as both heads were decked when I had them off to renew the head gaskets. I Also asked the machine shop to ensure they removed the same amount from each head to avoid any water bridge problems.
I'm away for a few days but will report back later with more results prior to pulling the engine. I'm going to pull the engine regardless because I'm not confident of getting a good job done on the cam carriers with the engine in the car.
Again thanks for your input and discussion.