Compression Testing vs. Leak Down Testing

[The Mic]

I have noticed that 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. Compression TestingTo 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. Leak Down TestingYou 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.

------------------------------------------------------------------------------------------------------------------------------------------------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 readingfrom 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 heara whooshing/howling by listening at the oil filler cap, you're losingcompression through the rings.

At the tailpipe: It is your exhaust valves if you can hear it (or evenfeel puffs on your hand) at the tail pipe.

At the intake manifold: It is your intake valves if you can clearlyhear 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.

Modified by S13GUY at 10:11 PM 2/1/2005
Modified by S13GUY at 10:12 PM 2/1/2005

[DriftinNC]

yeah, that should help clear up some confusion.... thanks for the info