How To Do a Compression Test (GM 4.8L, 5.3L, 6.0L)

Testing the engine compression on your 4.8L, 5.0L, or 6.0L engine is not hard to do.

A compression test will help you to determine the health of your engine by measuring the pressure of the air that gets compressed by the piston rings against the cylinder head valves.

A healthy engine will produce similar compression values on your compression tester on all 8 cylinders. An engine with internal problems (worn compression rings or worn cylinder head valves) will produce some or all cylinders with low or no compression.

This is a test that you can do (without having to take your pickup, van or SUV) to the shop and in this article, I'll show you how to do both a dry and wet engine compression test and then interpret the results.

OK, to make it as easy as possible to navigate this article, here are its contents at a quick glance:

  1. Symptoms of BAD Engine Compression.
  2. What Tools Do I Need?
  3. TEST 1: Dry Compression Test.
  4. Interpreting the Results of the Engine Compression Test.
  5. TEST 2: Wet Compression Test.
  6. Related Test Articles.

En Español You can find this tutorial in Spanish here: Cómo Probar la Compresión del Motor (GM 4.8L, 5.3L, 6.0L) (at: autotecnico-online.com).

Symptoms of BAD Engine Compression

Low compression in one or several engine cylinders will have a direct effect on idle quality. The symptoms you'll see will be:

  1. Rough Idle.
  2. Misfire Condition and Misfire Codes:
    1. P0300, P0301, P0302, P0303, P0304, P0305, P0306, P0307,P0308.

No compression in all of the cylinders will result in a Cranks but Does not Start Condition. The symptoms you'll see will be:

  1. No Start.
  2. Everything else works, for example:
    1. The fuel pump will activate, so you'll see fuel pressure at specification (if testing with a fuel pressure gauge).
    2. All of the COP coils will spark.
    3. If the COP ignition coils are sparking, then this indirectly proves that the crank sensor is OK too.
    4. The PCM will still activate all of the fuel injectors.

What Tools Do I Need?

The most important tool that you're gonna' need is a compression tester. You can either rent this bad boy from your local Auto Parts Store (Auto Zone, O'reilly, Advanced Auto Parts, etc.), or you can buy one online.

Since the spark plugs need to be removed, you'll need some of the following basic tools:

  1. Ratchet wrench.
  2. 5/8'' spark plug Socket.
  3. Extensions for the ratchet wrench.
  4. Motor oil (for the ‘Wet’ compression test part).
  5. Spark plug wire puller.

In case you're needing to buy a compression tester and want to save money by buying it online.. you can shop here: