Compression Test Procedure

• Use caution when removing secondary (spark plug) wires. First, twist the rubber boots to loosen them from the spark plugs.

• Clean around all the spark plugs with compressed air. Then remove all spark plugs, so that the starter can crank the engine easily.
• Block the throttle in the wide-open position. This can be done with a throttle depressor

• Insert the compression gauge into a spark plug hole.
• Crank the engine through at least four compression strokes.
• Check and record each pressure reading.

The gauge will move four times, or with all the plugs removed you can hear each compression
stroke as the compression in the cylinder being tested slows the engine.

• Connect a remote starter between the S terminal on the starter solenoid and the ungrounded battery post.
• The compression test is done with the ignition switch in the “on” position to prevent damage to the electrical system on some vehicles. Disable the ignition system by pulling the battery-todistributor wire on cars with electronic ignition. Follow the manufacturer’s instructions.


Interpreting Compression Test Results. If all cylinders are performing equally and engine performance
is acceptable, the engine passes the test. When compression test specifications are available,
they are only an estimate. If specifications are not available, locate the compression ratio in the specification manual and use the following
formula:

Compression Ratio × Atmospheric Pressure +

Atmospheric Pressure + 5 (Volumetric Effi ciency)

For example, to figure out the approximate compression on an 8:1 engine at sea level (14.7 psi atmospheric
pressure):

8.0 × 14.7 + 14.7 + 5 = 137.3 psi

Variations in compression among cylinders should be no more than 20%. When two cylinders

next to each other have low compression, a blown head gasket is usually indicated. One or several cylinders with low compression and no apparent pattern of loss often indicates burned exhaust valves with rough idling as a symptom. At higher rpm, the rough running from the burned valve may
disappear.


Wet Compression Test. When cylinders show poor results, perform a wet compression test.
• Squirt about a tablespoon of oil into each low cylinder. The oil makes a seal around worn rings, boosting the compression reading.
• When cylinder readings are low but increase to normal during a wet test, a piston ring problem
is indicated.


Running Compression Test
The running compression test, or volumetric efficiency test, is used when the cause of a cylinder misfire
cannot be pinpointed or when an engine lacks power. First, perform a regular cranking compression
test and record all of the results. Then install all of the spark plugs except for one and install a compression
tester in that hole. Start the engine, and depress the release pin on the Schrader valve to allow the reading to stabilize.


Snap Compression Test. The next step is to snap the throttle wide open and let the engine return to idle. When it is first snapped, the throttle plate is wide open while the piston speed is relatively low. This will result in a higher reading. The compression gauge will hold this reading until the Schrader valve is manually released.
Record your snap compression test readings. Typical readings will be around 80% of cranking compression
readings. Lower snap readings mean that the intake system is restricted. Higher readings indicate an exhaust restriction. Problems in only one cylinder point to a worn cam lobe, broken valve spring, etc. Problems in all cylinders can be traced to a restriction in the air intake system or a plugged exhaust pipe or catalytic converter.