How To Do An Engine Compression Test (GM 3.8L)

Testing the compression of each cylinder or a specific engine cylinder is becoming a standard test when troubleshooting hard to diagnose misfire conditions on the GM 3.8L V6 equipped cars and mini-vans.

A low compression condition in any of the 6 engine cylinders will cause a misfire condition and set misfire codes (if the vehicle is OBD II equipped: P0300, P0301, P0302, P303, P0304, P0305, P0306) that no matter what you replace, the misfire will continue to drive you crazy.

This article is part of a series of articles to help you to troubleshoot and repair a misfire condition on your GM (Chevrolet, Buick, Pontiac, Oldsmobile) 3.8L V6 equipped car or mini-van. The key article of the series is: How To Diagnose Misfire Codes P0300-P0306 (GM 3.8L).

En Español You can find this tutorial in Spanish here: Cómo Verificar La Compresión Del Motor (GM 3.8L) (at: autotecnico-online.com).

Tools You'll Need:

  1. Compression Gauge Tester.
  2. A Helper
  3. Pen and Paper

TEST 1: Dry Engine Compression

How To Do An Engine Compression Test (GM 3.8L)

Every engine cylinder, as you may already know, needs air, spark, and fuel to produce engine power. The other key component, that's never included into this formula, is the mechanical condition of the cylinder, specifically: the compression ability of the cylinder.

So then, this test will help you to verify that the specific cylinder you are testing, is contributing an adequate compression pressure to produce the required power output requirements.

This test article assumes that the engine starts and runs and that you're testing a misfire condition, but even if your engine doesn't start, this test will help you.

OK, without further ado, here's the test:

  1. 1

    It's important that the engine NOT be cold, and yet not be hot. So, if the engine has been running for an extended amount of time, let it cool down. If the engine is cold then I suggest you start'er up for about 15 minutes.

    Now, if the engine in your car or mini-van doesn't start, then this doesn't apply to you and let me tell you that the compression test (in this article) will still work.

  2. 2

    fuel injection must be disabled, so disconnect all 6 fuel injectors from their electrical connectors. This is important!

    In a few minutes, you're gonna' be cranking the engine and you don't want fuel being injected into the engine as you do the compression test.

  1. 3

    Disable the ignition system, so as not to produce spark during the tests.

    You can easily accomplish this by disconnecting the ignition module from its connector. This is also very important!

  2. 4

    Remove all 6 spark plugs. Be careful and don't drop any! This (them hitting the floor) could break the spark plug's ceramic insulator and this will cause a misfire!

  3. 5

    Thread the engine compression gauge into the spark plug hole for the number 1 engine cylinder. Hand tighten the compression gauge only! Do not use any type of tool to get it tight.

  4. 6

    When everything is all set up, have your helper crank the engine while you eye-ball the compression gauge (tester).

    Crank the engine till the needle on the gauge stops moving up. Once this point is reached, have your helper stop cranking the engine. Write this value down on a piece of paper. Repeat steps 1 thru' 6 on the remaining 5 cylinders.

  5. 7

    Now, look at all of the 6 compression readings that you just wrote down. They should be within 15% of each other. If you have never done this before and ‘within 15%’ has you scratching your head, don't worry, the next part will explain the whole thing.

The very first thing you'll notice (and kinda' comes as a shock) is that none of the compression readings are exactly the same! This is normal up to a certain point.

It's when they vary by a whole lot (more than 15%) that this becomes an issue. And so, in this section we'll interpret the numbers you wrote down, from testing the compression of each cylinder.

This is what you need to do:

  1. 1

    Grab a calculator and multiply the highest compression reading recorded by 0.15 (which is 15% in decimal form). So let's say, for the sake of this example, that the highest reading was 180 PSI (Pounds per Square Inch). Multiplying 180 by 0.15 gives us 27.

  2. 2

    Subtract 27 from the highest reading, which in my example is 180, and we get 153.

  3. 3

    So then, 153 PSI is the lowest possible compression reading that any one of the rest of the engine cylinders can have. Any compression reading below this and that engine cylinder will misfire.

To drive the math home, let me give you another more specific example, let's say that my 3.8L Buick LeSabre (or 3.8L Chevy Monte Carlo, or Pontiac Bonneville, or whatever) produced the following compression readings: Cylinder #1 = 165 PSI, Cylinder #2 = 30 PSI, Cylinder #3 = 155 PSI, Cylinder #4 = 170 PSI, Cylinder #5 = 165 PSI, Cylinder #6 = 155 PSI.

The next step is to do the math. Highest reading is: 170, so then I would multiply 0.15 x 170 and get: 25. Now I subtract 170 from 25 and get: 145 PSI. So 145 PSI is the lowest possible reading I should see in all of the compression test results. Now I know that Cylinder #2 (with 30 PSI) is the one causing the misfire!!



Buick Vehicles:

  • Century 3.8L
    • 1993
  • LeSabre 3.8L
    • 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005
  • Park Avenue (& Ultra) 3.8L
    • 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005

Buick Vehicles:

  • Regal 3.8L
    • 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004
  • Riviera 3.8L
    • 1993, 1994, 1995, 1996, 1997, 1998, 1999
  • Skylark 3.8L
    • 1993

Chevrolet Vehicles:

  • Camaro 3.8L
    • 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002
  • Impala 3.8L
    • 2000, 2001, 2002, 2003, 2004, 2005
  • Lumina (& MPV)
    • 1993, 1994, 1995, 1998, 1999
  • Monte Carlo 3.8L
    • 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005

Oldsmobile Vehicles:

  • 88 (& 88 Royale) 3.8L
    • 1993, 1994, 1995, 1996, 1997, 1998
  • 98 Regency 3.8L
    • 1993, 1994, 1995, 1996
  • Achieva 3.8L
    • 1993

Oldsmobile Vehicles:

  • Cutlass Ciera (& Cruiser) 3.8L
    • 1993
  • Intrigue 3.8L
    • 1998, 1999
  • LSS 3.8L
    • 1996, 1997, 1998, 1999
  • Regency 3.8L
    • 1997, 1998
  • Silhouette 3.8L
    • 1993, 1994, 1995

Pontiac Vehicles:

  • Bonneville 3.8L
    • 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005
  • Firebird 3.8L
    • 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002
  • Grand Am 3.8L
    • 1993

Pontiac Vehicles:

  • Grand Prix 3.8L
    • 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008
  • Trans Sport 3.8L
    • 1993, 1994, 1995