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Leak Down Tester
- Build and Use Your Own
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A leak-down tester is a measuring instrument used
to determine the condition of internal combustion engines by
introducing compressed air into the cylinder and measuring the rate
at which it leaks out.
Testing is done on an engine which is not running, and normally with
the tested cylinder at top dead center, although testing can be done
at other points in the compression and power stroke. Pressure is fed
into a cylinder via the spark plug hole and the flow, which
represents any leakage from the cylinder, is measured.
Leakage is given in wholly arbitrary percentages but these
“percentages” do not relate to any actual quantity or real
dimension. The meaning of the readings is only relative to other
tests done with the same design of tester. Leak-down readings of up
to 20% are usually acceptable while greater than that requires a
repair. Racing engines would be in the 1-10% range for top
performance.
While the leak-down tester pressurizes the cylinder, the mechanic
can listen to various parts to determine where any leak may
originate. For example, a leaking exhaust valve will make a hissing
noise in the exhaust pipe while a head gasket may cause bubbling in
the cooling system.
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A leak-down tester is essentially a miniature flow meter
similar in concept to an air flow bench. The measuring
element is the restriction orifice and the leakage in the
engine is compared to the flow of this orifice.There will be
a pressure drop across the orifice and another across
whatever leaks in the engine. |
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Since the
meter and engine are connected in series, the flow is the same
across both, the pressure drops will also be equal if, for example,
the size of the unknown leak would be the same as the known orifice.
This would give a reading of 50 which gives the same pressure drop
across each element(both the leak and the orifice). Conversely, if
there is no leakage there will be no pressure drop across either the
orifice nor the leak, giving a reading of 100 or 0% leakage.
There is no standard regarding the size of the restriction orifice
for non-aviation use and that is what leads to differences in
readings between leak-down testers from different manufacturers.
Most often quoted though is a restriction with a .040in. hole
drilled in it.
Some manufacturers use only a single gauge. In these instruments
maintaining the input pressure is (hopefully) maintained
automatically by the pressure regulator alone. Any error in the
input pressure will produce a corresponding error in the reading.
In instruments with two gauges the operator manually resets the
pressure to 100 after connection to the engine guaranteeing
consistent input pressure and greater accuracy.
Most instruments use 100psi as the input pressure simply because
ordinary 100psi gauges can be used which corresponds to 100% but
there is no necessity for that pressure beyond that. Any pressure
above 15psi will function just as well for measurement purposes
although the sound of leaks will not be quite as loud. An engine
pressurized to 100psi must be locked at exactly top dead center or
it will rotate under the pressure. This presents a serious danger to
the operator. Using less pressure is less dangerous and opens the
possibility to test at positions other than top dead center.
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How to Leak Down
a Barrel Valve |
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Block off all injector lines & returns except for hat nozzles
Connect dual gauge tester to outlet of compressor
Set Air Compressor to read 100psi on 1st gauge
Connect tester to inlet of barrel valve
Read % on 2nd gauge
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