How to Test Engine Block Compression

Check your engine with a compression gauge.

When the gasoline explodes in a cylinder and moves the piston, worn rings or valves or a bad head gasket allows part of the energy of the explosion to escape, causing a loss of power and poor fuel economy. This means bad compression. You can check the compression of an engine yourself with a compression gauge. Ask a friend to help you, because you must engage the starter while working under the hood.

Things You'll Need

• Spark plug wrench
• Compression gauge
• Wrench
• Pen or pencil
• Note pad

Instructions

1. Remove all the spark plug wires and then remove all the spark plugs form the engine. Be careful not to allow dirt to fall into the spark plug holes.

2. Disable the ignition by pulling the main coil wire from the distributor or the fuse or main ground wire from an electronic ignition.

3. Insert the input hose from the compression gauge into one of the spark plug holes and tighten it with the wrench.

4. Have your friend engage the starter and crank the engine for about five seconds.

5. Note the reading on the compression gauge and write it down. Typically the reading should be about 140 to 160 psi (pounds per square inch).

6. Perform the compression test on each spark plug hole and record the readings. All readings should be within plus or minus 10% of each other.

Tips & Warnings

• If only one cylinder reads low, you could have a bad valve. With two reading low you could have a bad head gasket. All low indicates bad piston rings meaning you need an engine overhaul.

• Compression tests work better on a warm engine because the rings seal better,but you can burn your fingers removing the plugs. A compromise would be to run the engine for a minute or two before checking the compression.

References

• AA1Car: Engine Compression Testing