How to Perform a Leak-Down Test

When your street or race engine starts showing signs of "lack-of-power", a common and yet simple test that can be performed is a "Leak Down" test. A "Leak Down" test pinpoints where you are losing compression and exactly how much before you open up your engine. The test involves a special gauge and adding compressed air to measure the percentage of leak in the cylinder. 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.

There are many manufacturers of these gauges and they are available at just about any auto parts house. Depending on the particular "Leak Down" test gauges you get, the actual test will go something like this:

    1. Take out the spark plugs.

    2. Bring each cylinder to TDC (top dead center) so that all it's valves are closed.

    3. Screw the gauge adapter into the spark plug well and connect the gauge. The gauge 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.
    • At the crankcase: Remove the oil filler cap. If you can clearly hear a whooshing/howling by listening at the oil filler cap, you're losing compression through the rings.
    • At the tailpipe: It is your exhaust valves if you can hear it (or even feel puffs on your hand) at the tail pipe.
    • At the intake manifold: It is your intake valves if you can clearly hear a whooshing/howling by listening at the throttle body/intake manifold.

    6. Repeat steps for each cylinder.

If the results are inconclusive, it could be a blown head gasket. Symptoms of a blown head gasket are: reduced power, white or blue smoke out the tailpipe, oil in the coolant or coolant in the oil. You can check for a blown head gasket by removing the coolant cap and watching the coolant level when you add the compressed air. If it rises or you see bubbles, you have a leaking head gasket. Call our Technical Department for more information or details at (928) 855-6341.

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