TEST 2: Wet Compression Test

How To Check Engine Compression (GM 3.5L, 4.2L)

A ‘Wet’ compression test will help you to find out if the low cylinder pressure or pressures you recorded in the ‘Dry’ compression test are caused by worn piston rings or worn cylinder head valves.

What you're gonna' do is add about 2 tablespoons of oil to the cylinder (that recorded no or low compression in the previous test) and then check its compression once again.

What will happen is that if the low compression values is due to worn piston rings, the compression value will go up from the previous one you got doing the Dry compression value.

If the compression value does not go up (from the previous one), then you'll know that the problem lies in the cylinder head valves.

OK, this is what you need to do:

  1. Add a small amount of engine oil to the cylinder that reported low compression or no compression in the ‘Dry’ compression test
    1. The amount should be about 1 to 2 tablespoons of oil.
  2. Install the compression tester onto the cylinder.
    1. Do not use any type of tool to tightened the compression tester. Hand tight is fine.
  3. When all is set up, have your helper crank the engine.
  4. You'll get one of two results, either the compression value will go up (from the one you recorded before) or it will stay the same.

Let's take a look at your test results:

CASE 1: The compression value shot up. This tells you that the piston compression rings are worn out and thus the problem is in the bottom end.

CASE 2: The compression value stayed the same. This confirms that the problem is in the cylinder head valves.

Engine Compression Test Summary

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Buick Vehicles:

  • Rainier 4.2L
    • 2004, 2005, 2006, 2007

Chevrolet Vehicles:

  • Colorado 3.5L
    • 2004, 2005, 2006
  • TrailBlazer 4.2L
    • 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009

GMC Vehicles:

  • Canyon 3.5L
    • 2004, 2005, 2006
  • Envoy 4.2L
    • 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009

Oldsmobile Vehicles:

  • Bravada 4.2L
    • 2002, 2003, 2004