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Compression Testing vs. Leak Down Testing |
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There has been great confusion encountered
regarding compression tests and leak down tests. A compression test is
typically done prior to a leak down test and is an easy way to determine
the internal health of your engine. You can have a qualified mechanic
do this or you can do it yourself with the proper equipment in about 10
– 15 minutes. A leak down test may also be called a cylinder
differential test. This test will allow you to diagnose where and
exactly how much compression you are losing prior to tearing down your
engine. Again, a qualified garage/mechanic can perform this test, but
be very specific when requesting this, since it is frequently confused
with a compression test. |
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You can do it yourself with the proper tools.
Listed are just some of the tools we offer/recommend. |
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Compression Testing |
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To do this you will
need a compression gauge, a spark plug socket and socket wrench. You
will want to remove all four spark plugs and inspect them carefully for
any poor signs of use. Signs of fouling may signify a bad valve seal.
Signs of oil on the plug could indicate a ring problem. You will then
screw the compression gauge into the plughole. By depressing the gas
pedal to the floor, you will open the throttle plate and want to keep it
fully opened for as long as you crank the motor. By cranking the motor
we mean, turning the ignition to the ON position and holding it there
for 4 full revolutions. You will see the needle jump slightly less on
each rotation. It may take more than 4 revolutions to get a stable
reading. The last number the gauge jumps to is the compression for that
cylinder. Repeat this for each cylinder. You will want to make sure
your battery is fully charged or you will get lower readings due to weak
cranking.
Warning: Please refer to your owner's manual to insure the correct way
to prevent any ignition spark or fuel leakage prior to performing this
test. |
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Leak Down Testing |
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You will need a
constant supply of compressed air and a leak down gauge to perform this
test yourself. One face of the gauge measures the pressure of the
compressed air that is being forced into the cylinder, while the other
measures the percentage of this pressure that is being lost. A knob
allows you to control the pressure going in so you get even results.
You begin by taking out the spark plugs and bringing the piston (each
cylinder at a time) to TDC (top dead center) so that all valves are
closed. You will then connect the gauge to the spark plug well and the
to the air hose. Apply the compressed air and modulate the knob to get
a steady reading from both gauge faces. Use the same pressure on all
cylinders. You will want your compressor full and stopped to achieve
consistent readings for each cylinder. You will then listen to where
you hear the compressed air if it is escaping. If you hear
whooshing/howling at the oil filler cap, you are losing compression
through the rings. If you hear it or even feel puffs at the tail pipe,
it is your exhaust valves. If you can hear near the throttle
body/intake manifold, it is your intake valves. If the results are
inconclusive, it is probably your head gasket.
Warning: Please refer to your owner's manual to insure the correct way
to prevent any ignition spark or fuel leakage prior to performing this
test. |
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