How To Do a Compression Test on an Engine

By | September 19, 2022

How To Do a Compression Test
Doing a compression test is an easy task for a DIY mechanic and can show if you have bad piston rings or valves.

Bad rings or valves can cause a leak to occur around them, which instead of sending a piston downward, the air goes around the piston.

This will give an engine a lack of power and need new rings and possibly valves.

The first step to finding out if it is the rings or valves that need to be replaced is to do a dry and wet compression test.

Tools Needed for an Engine Compression Test

  • Engine Compression Tester
  • Spark Plug Removal Wrench
  • Small Amount of Engine Oil

How To Do a Compression Test on an Engine

  1. Remove the spark plug on the cylinder to be tested.
  2. Insert the Compression Tester Tool.
  3. How To Do a Compression Test Step 1

  4. Turn the engine over 7-8 times and record the pressure.
  5. Remove the Compression Tester Tool and insert roughly 10cc of engine oil into the cylinder.
  6. Insert the Compression Tester Tool again.
  7. Turn the engine over 7-8 times and record the pressure again.
  8. Do I Have Bad Valves or Bad Piston Rings Compression Test Wet

How To read Engine Compression Test Results: Bad Rings or Valves?

  • If the Compression goes up, the rings are leaking.
  • If the Compression stays the same, the valves are leaking.

Summary
Doing an engine compression, while simple, can give a lot of information.

Knowing if the engine has bad valves or bad rings can save a lot of time and money before starting a tare down.

If the rings are worn and gone bad, they will need to be replaced and often send white smoke out of the exhaust.

If the rings are good, there is no need to tare down the engine, which a simple compression test will show.

When the valves are bad only the engine heads will need to be removed.

A compression test may be old school, but it is a tried method that works.

Have you done a compression test on an engine? Let us know your thoughts below.

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