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What is a Leak Down test?

It is also sometimes called a cylinder differential test .  A leak down
test pinpoints where you are losing compression and exactly how
much. You should do a compression test first.

Click here for the Compression Test VFAQ.

Why get a Leak Down test?

Doing a leak-down test diagnoses where you're losing compression
before you open up your engine.

If you did a compression test and your results are:

then your engine is running less than optimally and you should consider
rebuilding it. You can certainly run for a good long time with these
conditions, but you are not doing yourself or your car any favours and if you
put it off for too long you might not have the same choices and things
could end up being even more expensive.

Where can I get a Leak Down test done?

Many garages don't even know what a leak down test is. They confuse it
for a compression test, which anybody can do in 10 minutes with a $20
gauge. If you are reading this, you've probably already done several
compression tests, and now you just want to know whether it's your rings or
your valves or your head gasket.

Call around to find a shop that has a leak down gauge. Be very, very
specific that you do not need a compression test, you want a leak down
test, the one that uses the compressed air. I actually lost my time showing
up at 2 garages who swore over the phone they could do a leak down test
and when I got there they were expecting to do a compression test.

Can I do it myself?

Sure, but you need to have compressed air available, as well as the
special gauge. The gauge looks like this:

One face of the gauge measures the pressure of the compressed air
that you are forcing into the cylinder, and the other face measures the
percentage of this pressure that is being lost. There is a knob on the
gauge that lets you control the pressure going in so you get even results.

I understand the gauge is not exactly cheap and can be gotten from
both aviation and automotive sources.

How to do a Leak Down Test

1. Take out the spark plugs.

2. Bring the cylinder TDC (top dead center) so that all the valves are closed.

3. Remove the skinny extender, screw the gauge adapter into the spark plug
well and connect the gauge. The guage has a fitting to connect the
extender hose to the spark plug well, and another to connect the air hose.

4. Apply the compressed air and modulate the knob to get a steady reading
from both faces on the gauge. Use the same pressure on all cylinders.

5. LISTEN to where you can hear the compressed air.

6. Repeat for each cylinder. Do 1&4 or 2&3 together, as they are in the TDC position in pairs.

7. Good luck :o)

Information Source: www.geocities.com/dsmgrrrl/FAQs/leakdown.htm