RE>Leak down test instrument

[geo3@earthlink.net]

Johnston, James W <james.w.johnston@intel.com> wrote:

>Correct me if I am wrong, for I am surely not a professional, but what I
>know of leakdown tests in my experience is similar to a compression test
>where you crank the engine and watch the pressure in the cylinder slowly
>dissapate to see how well it holds the pressure...why would you need to
>pressurize it externally with compressed air and that special 'leakdown
>tester'?  wouldnt a compression tester be sufficient?

A compression test will tell you, well, compression.  Take a cylinder, fill it
with air, compress it using the piston and there is your reading.  Valves are
continually openning and closing with the engine sucking, squishing, *not*
banging, and blowing.  It will tell you some of the story of the condition of
the valves, their seats, rings, cylinder bores, etc.

A leakdown test cannot be done while cranking because the valves will keep
openning and closing. :-)  So, you must pressurized a sealed cylinder (both
valves closed) and let it leak down over time.  This will tell you a bit more
about the condition of the above parts.  You can also get an idea whether leak
down is due to rings or valves by putting oil in the cylinder and seeing if
the leakdown improves.  If it does, the rings are bad.  If it doesn't, either
the rings are *really* bad, or the valves/seats are bad.

A good engine will leak down very little over several hours.

George Roffe

mail2web - Check your email from the web at
http://mail2web.com/ .