Mechanics

Compression Test

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Step by step instructions on how to check the compression in a cylinder of an internal combustion engine. To perform an engine cylinder compression test follow the steps below.

Difficulty Scale: 3 of 10

Tool and Supplies Needed

  • Compression gauge
  • Spark plug socket and ratchet
  • Protective eyewear and gloves
 

Compression Gauge
Compression Gauge

Step 1 - Remove ignition coil connector(s) or ignition system, fuel pump fuse to disable power to either system. This is to have the engine not start when cranked over, which is necessary for this test.

Step 2 - Remove the spark plug in question and insert the compression gauge, most gauges have a hose attachment which is then connecting to the gauge.

Step 3 - After the compression gauge has been inserted, crank the engine over for about five seconds. (Note: use about the same five second test for the remaining cylinders.)

Step 4 - Remove the compression gauge and reinsert the spark plug, and continue onto the remaining cylinders.

Step 5 - Compare all cylinder compression readings, typical compression readings are between 125 p.s.i. and 160 p.s.i. Cylinder readings should be within about 7% of each other.

 

Helpful Information

An engine depends on an equal amount of compression in each cylinder to operate smoothly. If poor compression exists in one or more cylinders it can cause a rough idle condition and low power. A compression test can be performed to diagnose the cause of a low cylinder, such as wear or internal damage. A compression gauge is needed to test the engine cylinders since the pressures are in excess of 100 psi. 

There are two types of gauge styles, one threads into the spark plug hole which is more accurate. The other style of gauge is constructed with a rubber plug that is designed to be pressed against the spark plug hole, this style of gauge is difficult to use and may not be accurate.

If low compression exists, further disassembly and inspection will be required to find the cause. Possible causes for a low compression condition are: burned intake or exhaust valve, broken piston or piston ring, broken valve spring or a blown head gasket.

If no or little compression exists additional tests will be needed. The most common reason for an engine to lose compression is a timing belt or timing chain failure.

If low or no compression exists remove the oil fill cap and observe camshaft rotation when the engine is cranked over. If no rotation exists the timing belt or chain has failed. If the engine has a timing belt which is not visible remove the upper bolts to the timing cover to gain visual access to the belt, recheck cam rotation by cranking the engine over. Sometimes a timing belt or chain can jump out of time causing the camshaft to lose correlation with the crankshaft and therefore causing low compression. The best test for this condition is to remove the timing belt/chain cover and inspect timing marks.

To check compression you must first disable the ignition system to keep the engine from starting. Locate the ignition coil and disconnect it, or unplug the ignition module connector. To test if the ignition is disabled crank the engine over, the engine should not start.

Optional compression testing methods include an electric starter tester. This test measures the battery amperage draw to each cylinder while the engine is cranking. It will show which cylinders have good compression and which have the least compression. Each manufacturer produces specific compression ratios for their engines to control the power output and emissions. If high compression exists it can cause a problem known as pre ignition or detonation. A compression test can detect internal combustion malfunctions such as, bad valves, bad piston rings, or excessive carbon build up. These problems if left un-repaired can cause more extensive engine damage and even catastrophic failure.

Best Practices

  • Replace spark plug at the test of a full compression test if needed.
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AUTHOR


Written by
Co-Founder and CEO of 2CarPros.com
35 years in the automotive repair field, ASE Master Technician, Advanced Electrical and Mechanical Theory.


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Article first published (Updated 2013-11-14)