Guilty as charged, Bud. 8^) If anyone's interested, here's the complete
text of the note I posted in November...
The following is what my VW Factory Service Manual (a Bentley manual, by
the way) has to say about compression testing. For a VW, but...
compression is compression!
"Low compression is evidence of poorly sealed combustions chambers.
Generally, compression pressures which are relatively even, but below
acceptable specifications indicate worn piston rings and/or cylinder
walls. Low, but erratic values tend to indicate valve leakage. Dramatic
differences, such as acceptable values in some cylinders and very low
values in one or two cylinders are the sign of a localized failure,
probably of the head gasket. There are two more tests which can further
isolate the problem.
Wet Compression Test
To analyze poor compression and further identify the source of the
leakage, repeat the compression test, this time with about a tablespoon
of oil squirted into each cylinder. The oil will temporarily help seal
between the piston rings and the cylinder wall, practically eliminating
leakage past the rings for a short time. If this test yields higher
compression readings than the dry test, the difference can be attributed
to leakage between the piston rings and cylinder walls. Little or no
change indicates other leakage, probably from the valves or a failed
head gasket.
The most conclusive diagnosis of low compression requires
The Leak-Down Test
Using a special tester and a supply of compressed air, each cylinder is
pressurized. The rate at which the air leaks out of the cylinder, as
well as the sound of the air escaping, can more accurately pinpoint the
magnitude and source of the leakage. Any engine compression diagnosis
which will require major disassembly should first be confirmed by the
more accurate leak-down test. Because the test requires special
equipment and experience, it may be desirable to have it performed by a
Volkswagen dealer or other qualified repair shop."
Thought this might be of some benefit to those of you in the process.
--
Michael Ferguson
Bud Rolofson wrote:
>
> The wet test is different than a leak-down test.
>
> >From the archives (Michael Ferguson I believe):
>
> The Leak-Down Test
>
> Using a special tester and a supply of compressed air, each cylinder is
> pressurized. The rate at which the air leaks out of the cylinder, as
> well as the sound of the air escaping, can more accurately pinpoint the
> magnitude and source of the leakage. Any engine compression diagnosis
> which will require major disassembly should first be confirmed by the
> more accurate leak-down test. Because the test requires special
> equipment and experience, it may be desirable to have it performed by a
> dealer or other qualified repair shop."
>
> ____________________Reply Separator____________________
> Subject: Compression test/"leak-down" test
> Author: Pete & Aprille Chadwell <dynamic@transport.com>
> Date: 02/18/1999 3:58 PM
>
> Bud Rolofson wrote:
>
> >A dry test can tell you: that you might have a faulty head gasket if two
> >adjacent cylinders have low readings, or if the pressure builds up
>erratically
> >(as you crank the engine) the problem could be burned valve seats or sticky
> >valve guides.
> >
> >A wet test is done to help diagnose WHY your compression is low, it is not
> >done
> >to confirm low readings on the dry test.
> >
> >e.g. On a wet test, if the compression increases to nearly normal then the
> >rings, piston, and cylinder (for that cylinder) are probably in need of
> >service.
>
> Is this "wet test" the same as a compression "leak down" test?
>
> Can anyone describe how a "leak-down" test is performed and how it is
> different from a "wet test" if it IS different?
>
> I've done regular compression tests and I've got those handled just fine.
> But these others have me a bit puzzled.
>
> Thanks,
>
> Pete Chadwell
> 1973 TR6
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