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
Vernon CT
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