How to Diagnose Misfire Codes
P0300-P0306 (GM 3.8L)

What Causes a Misfire Condition? (Continued)

Fuel System: The most common type of failure, in the fuel system of the 3.8L fuel injected engine, is one or more fuel injectors going BAD. Here are some more specifics:

  1. BAD Fuel Injector(s):
    1. Injector is fried internally and does not spray any fuel.
    2. Injector is clogged and does not sprays enough fuel.
    3. Injector is clogged/dirty and spray fuel in an incorrect spray pattern.
    4. Fuel injector is not being activated by the PCM.
    5. Fuel injector is not getting power (12 Volts).

Engine Mechanical Condition: An engine cylinder that is not producing enough compression, even it has the spark and fuel, will cause a misfire condition that will set a misfire code. Another thing that will cause a misfire (normally at idle) is vacuum leaks coming from a large vacuum hose or the intake manifold gasket.

  1. Low or No Engine Compression:
    1. One or two cylinders have low or no compression.
    2. Low or no engine compression will be the result of:
      1. Worn cylinder head valves.
      2. Worn piston rings.
      3. Blown head gasket.
  2. BAD Intake Manifold Gasket:
    1. The intake gaskets are leaking coolant and this is a sure sign of gasket failure that can and does affect idle quality.

What Tests Can I Perform
to Find the Cause of the Misfire Condition?

It is possible to find out exactly what's causing the misfire condition and/or misfire code on your 3.8L V6 equipped GM vehicle.

The very first thing that has to be done, is to identify the cylinder that is misfiring (or dead), then you can go about doing specific tests to find out if it's being caused by a lack of spark or fuel or enough cylinder compression.

The following is a list of tests that may need to be done to find the cause of the misfire problem. Normally, you'll only perform some of these tests:

  1. Read the diagnostic trouble codes with a scan tool. On OBD II equipped vehicles, you should see a specific cylinder misfire code that will tell you what engine cylinder is the one being affected.
    1. There's a good chance, that even tho' your vehicle is OBD II equipped, the PCM will not set a specific misfire trouble code, when this happens, you'll need to do a cylinder balance test (you can find it here: How to Do a Cylinder Balance Test (GM 3.8L V6)).
  2. On older vehicles (that are not OBD II equipped), you'll need to do a cylinder balance test to find out the missing (dead) cylinder. You can find this test here: How to Do a Cylinder Balance Test (GM 3.8L V6).
  3. Test the ignition system first. Which in the 3.8L V6 engine includes:
    1. Troubleshooting the spark plug wires.
    2. Troubleshooting the 3 ignition coil packs sitting on top of the ignition control module (ICM).
    3. Troubleshooting the spark plugs.
    4. Troubleshooting the ignition control module (ICM).
    Why test the ignition system first? Because the majority of misfires are due to an ignition system component having failed.
  4. Testing the compression of all engine cylinders.
  5. Testing the fuel injectors
  6. Check for vacuum leaks.

It seems like a long and complicated list... doesn't it? Well, don't worry... I'm going to show you how to test all of these systems and/or components so that you can find out what is the component that has gone BAD and is causing the misfire code(s) (P0300, P0301, P0302, P0303, P0304, P0305, P0306) or the misfire condition. You'll find the links to all of these tests in the next page, at the end of the article.

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

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