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How to Compression Test Your Car's Engine


How to Compression Test Your Car's Engine

Compression Tester

Michele Hamer
Checking your engine’s compression should be part of your regular maintenance routine because it will give you an indication of your engines overall health and detect internal engine malfunctions such as bad valves, piston rings or excessive carbon buildup before they cause irreparable damage. It’s a relatively easy test to do and all you’ll need is some labels, a helper, paper and pencil, and decent engine compression kit, about $30, that should include adapters for 14mm long reach and standard spark plug threads.
Difficulty: Easy
Time Required: 30 Minutes

Here's How:

  1. Let the car run for about 5 minutes, you always want to compression test a warm engine for best results.
  2. Disable the ignition system so the engine does not fire up during testing.
  3. Label the spark plug wire with the same number as you label the spark plug hole, 1 through 6 for example, before removing all the spark plugs.
  4. Insert the compression tester into the cylinder spark plug hole labeled #1 and tighten by hand only.
  5. Have your helper crank the car’s engine 7 to 10 times continuously to obtain the most accurate reading on the compression gauge.
  6. Record the reading for the cylinder labeled #1 and remove the tester from the spark plug hole.
  7. Make sure that the pressure gauge returns to zero before continuing testing and recording the rest of the cylinders while the engine is still warm.
  8. Clean and replace the spark plugs with the correct spark plug wire.


  1. The compression test is not so much about the absolute level of compression; it is the difference in the compression between each of the cylinders. If all cylinder readings are within 10 percent of each other, your engine compression is considered optimal. However, if any of the cylinders compression vary 10% or more from each other, a problem may exist and further inspection of the engine should be completed.

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