How to Troubleshoot Misfire Codes (GM 3.1L, 3.4L)

Testing a misfire condition or misfire codes (P0300, P0301, P0302, P0303, P0304, P0305, P0306) on a 3.1L or 3.4L V6 Gm car or mini-van can seem like a really difficult task. You could be asking yourself questions like: Where do I Start? What Do I test? Well, in this article I'll try to shed some light on troubleshooting a misfire condition on your 3.1L or 3.4L GM car or mini-van.

What I've done, to better help you, is to write an overview of the testing tips and techniques that are performed to get the bottom of a misfire code and/or misfire condition... and more importantly, where to find the test articles you'll need to get to the bottom of the misfire problem.

Here are the contents of this article at a quick glance:

  1. What Is a Misfire Condition?
  2. What Causes a Misfire Condition?
  3. My Vehicle Has OBD II
    Where Do I Start My Misfire Diagnosis?
  4. What Does the Misfire Code Tell Me to Replace?
  5. My Vehicle Does NOT Have OBD II
    Where Do I Start My Misfire Diagnosis?
  6. Where Do I Find the Test Articles?

What is a Misfire Condition?

A misfire condition is a condition in which the engine in your car is not firing on all cylinders. When this happens, you'll notice some of the following Symptoms of a misfire:

  1. The Check Engine Light (CEL) will be lit on your instrument cluster.
  2. If your vehicle is OBD II equipped, you'll have one or more misfire codes (P0300, P0301, P0302, P0303, P0304, P0305, P0306) stored in the computer's memory.
  3. Sometimes, even if your vehicle is OBD II equipped, the Computer does not register any misfire codes, even tho' the car or mini-van is suffering a bona-fide misfire condition.
  4. Lack of power upon acceleration.
  5. Smell of unburned gas exiting the tail pipe.
  6. Rough idle and may stall.
  7. Cranks but does not start or finally starts after a very long extended cranking time.
  8. Will not pass the emissions tests (smog check).
  9. Bad gas mileage.

What Causes a Misfire Condition?

A misfire (or dead cylinder) is usually caused by a problem in the ignition system. Specifically, a lack of spark reaching the spark plug... but not always. Let's take a brief look at the different things that can cause a misfire:

Ignition System: The ignition system on the 3.1L and 3.4L GM V6 engines is made up of 3 coil packs (sitting atop the Ignition Control Module), spark plug wires and spark plugs. If any of these components goes BAD (and they do) and they stop creating or transmitting spark, then you'll end up with a misfire condition.

The components that are usually behind most ignition system misfires are:

  1. BAD spark plug wires.
    1. They usually don't fail all at once. For the most part, you'll have one or two spark plug wires that simply stop transmitting the spark from the coil pack to the spark plug.
  2. BAD ignition coil pack.
    1. Each ignition coil has two towers and generally one of them stops creating spark, although it's not uncommon for both to stop creating spark.
  3. BAD ignition control module (ICM).
    1. The ignition module usually fails in one of two ways: It either stops activating all of the 3 ignition coils sitting on top of it or it stops activating 1 of the 3. Having one ignition coil not creating spark will cause a misfire condition.
  4. BAD spark plug.
    1. spark plugs don't last forever and are gonna' wear out eventually. What agravates the problem is if the engine in your car or mini-van is burning oil. If this is the case... one or several spark plugs become carbon fouled and the air gap (between the center electrode and the side electrode) closes. This results in a no spark condition for that or those specific cylinders.

Fuel System: fuel system induced misfires (dead cylinders) are rare.. but they do happen. The usual problem areas are:

  1. BAD or clogged fuel injector.
    1. They usually don't fail all at once. For the most part, you'll have one fuel injector that has fried internally and stops spraying fuel.
  2. Electrical problems.
    1. By electrical problems I mean problems in the circuits that feed it the fuel injector with power or it's On/Off signal. Or a BAD Computer (since the Computer -PCM- is the one that activates them).

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 may or may not set a misfire code) or a rough idle condition. Most folks refer to this as a dead cylinder. Another thing that will cause a misfire (normally at idle) is vacuum leaks coming from a large vacuum hose or the intake manifold gasket.

In the next section, let's talk about where to start your misfire diagnosis/troubleshooting...