Why would a compression test show all cylinders good, but a leak-down shows one cylinder with high leakage? Thanks, Wayne
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Re: Leak-down test vs. compressions test
It shouldn't... If the compression test was done properly, it should have indicated low compression on the cylinder the leak down test spotted as not holding pressure.
Now, if the leak down rate isn't 'sky high' and the mechanic who did the compression test didn't hold the cylinder closed for very long (just bumped it over and watched the compression gauge peak), I guess you could get those results.
Did the leak down test point to faulty piston rings or to faulty valves? -
Re: Leak-down test vs. compressions test
A leak-down test is static and only shows the compression ring seal at one piston position in the cylinder bore.
A compression test is dynamic and is an indication of the over-all seal of the rings throughout the piston travel in the cylinder bore.Comment
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Re: Leak-down test vs. compressions test
Jack and Wayne-----
Not always. A compression test is a rather crude test for cylinder sealing. It will yield "mixed results" and, sometimes, inaccuracies.
A leak-down test is a much more accurate and reliable means of assessing cylinder sealing. As a matter of fact, this is the only type of test that the FAA allows for use on piston aircraft engines. A compression test is not an allowable methodology.In Appreciation of John HinckleyComment
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Re: Leak-down test vs. compressions test
Wayne,
Some key questions:
1. At what pressure did you conduct the test? 100? 90? 80psi?
2. Was the engine hot? (warm, within 45 min. approx., of shutdown)
3. What were the per cent leakage rates on all 8 cylinders?
4. Was the throttle tied wide-open?
6. Where did you hear the leakage on the "bad" cylinder? Crankcase? Carb? Exhaust?
Like Jerry said, the comp test has the rings in motion, thereby providing the best seal that they can make. Sometimes, in a (static)leakdown test, you have to "toggle" the piston until the optimum seal is made. Higher test pressures yield better test results, because 100 psi forces the top ring to seal better than lower pressures.
JoeComment
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Re: Leak-down test vs. compressions test
Joe-
I conducted the leakdown test at 100 psi. The engine was stone cold, has not been run for two weeks. The percentages were: 18, 65, 10, 8, 18, 10, 18, 10. In all cases, I detected no air leakage in the intake, exhaust, or bubbles in the coolant. On all eight tests, the air leakage was noticeable in the oil fill tube. I heard air leakage on all eight tests, but it was most noticeable on cylinder #2 where the % leakage was 65%. Leakddown tests were run with piston at TDC. Thanks, WayneComment
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Re: Leak-down test vs. compressions test
You may have an issue with #2, possibly a stuck ring, but rerun the test with the engine hot. The numbers should be a little less. Start with #2, then the other seven, then #2 a second time.
Multiple leakdown tests can have varying results, so run any suspicious cylinders at least twice and be sure the adapter is properly sealed to the spark plug bore.
DukeComment
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Re: Leak-down test vs. compressions test
Wayne,
Run the test hot. Move the questionable piston back and forth near TDC until the ring seats fully. Try the test on number 2 with the piston at other positions in the bore. If you get a better seal elsewhere, then the leakage is probably due to a scored cyl wall rather than a bad ring.
65% is significant!!! Your #2 plug should be oil fouled with such bad blowby! Do you have a dead miss in #2? What was the initial cause of your diagnostic? Did you suspect a problem before testing??
JoeComment
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Re: Leak-down test vs. compressions test
Joe-
Will repeat the test hot. I have done the leakdown test several times, but the engine has been cold in each case. The car runs well with no noticeable miss. All the plugs are dry, and look the same under a magnifying glass. The initial cause for diagnosis was high oil comsumption, a quart about every 500 miles. Thanks for all your help, WayneComment
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Re: Leak-down test vs. compressions test
ran a comp. test on a 455 olds all appeared fairly good % wise one cyl off removed heads anyhow---found quarter size hole in one piston last time i used comp. test went to leakdown
i prefer to do leakdown with the piston at bdc for 2 reasones---1 safety as they only way i have found to hold the motor from spinning is to lock up the gear box in reverse and a fwd gear---if i try to hold it by the dampner bolt i have had it come loose even after tightening it to 80ftlbs---reasone 2 is if there is a hairline crack in the mid or lower portion of the cyl it will show up---also give some indication as to excessive bore taper---good luck,philComment
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the most wear is at the top of the cylinder
that is why you leak down at TDC. if the cylinder is a exact TDC the air will not push the piston down. they sell a "whistle" that goes in the spark plug hole that tell you when you are at exact TDCComment
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Re: Leak-down test vs. compressions test
You can also run a cylinder balance test.
With the engine idling, disconnect each plug wire one at a time. If disconnecting #2 does not result in as much idle speed drop as the others, it is not doing its fair share of work.
Disconnecting plug wires with a point type ignition should not cause any harm, but it can damage electronic ignitions, in which case you should rig up a method to short the plug wires one at a time - like sliding the boots up each wire at the cap so you can touch a shorting lead to each wire terminal.
DukeComment
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Re: the most wear is at the top of the cylinder
Clem,
I don't think the whistle is that accurate. The piston blows air and makes a sound through the whistle as it nears TDC. That whistle is only good to let you know the piston is on the compression strike rather than the exhaust. You know the only way to determine true TDC is with the head off and a dial indicator on the piston. (or a piston stop through the spark plug hole with a degree wheel to divide the forward and reverse numbers in half).
VerneComment
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Re: the most wear is at the top of the cylinder
backwards on taper but cylinder cracks from "seasoned rusted" blocks will not show leakage at TDC maybe they should be checked at both TDC and BDCComment
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it will work for getting close enought to keep
the air from pushing the piston down when you do a leak down. it does take some practice to get it right.. you just have to get the con rod straight up and down in relation to the crank throw.Comment
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