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Compression Testing vs. Leak Down Testing

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.

You can do it yourself with the proper tools. Listed below, are just some of the tools we offer/recommend:

  • Cylinder Leak Down Tester, see Part# 1093078

  • Compression Tester, see Part# 1093075

  • Haynes Repair Manual (79-95), see Part# 1093039

Compression Testing:

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.

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 Testing:

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.

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.