How To Test Misfire Codes (GM Quad-4 2.3L, 2.4L)

I can still remember the first time a Quad-4 equipped GM came into the shop (where I worked) and thinking what a pain in the neck it would be to work on it... after this first impression and as the years and experience accumulated I can tell you that although it's not easy working/troubleshooting these vehicles it's also not that hard or difficult.

One of the most common problems that you (or I) are gonna' face with a Quad-4 equipped vehicle is a misfire condition. Although it stinks that your Quad-4 GM car is experiencing a misfire condition... the cool thing is that there's a method to the madness of troubleshooting it.

In this tutorial, I'm going to explain in some detail the most common causes of misfires and misfire codes (P0300, P0301, P0302, P0303, P0304) and more importantly, I'm also gonna' offer you a simple diagnostic strategy that I'm certain will help you ‘nail down’ the cause of the misfire condition, misfire code, or rough idle condition your 2.3L or 2.4L Quad-4 GM car is experiencing.

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

  1. What Is a Misfire Condition?
  2. What Causes A Misfire Condition?
  3. How Can I Test The Misfire?
  4. What Tools Do I Need To Test The Misfire Code(s)?
  5. More Quad-4 Tutorials.

Let's get started by jumping right into the next subheading...

What Is a Misfire Condition?

As you're already aware, the 2.3L or 2.4L Quad-4 engine has four cylinders. In a nutshell, each one of those four cylinders needs three things to produce power and they are: fuel, spark, and air (typically compression).

When your Quad-4 equipped GM vehicle is suffering a misfire... it's because one of those three things is missing from one of the four cylinders. Simplified: A cylinder is misfiring because it's missing fuel, or spark, or air (compression).

Since the 2.3L and 2.4L engines have only 4 cylinders, having just one of those cylinders misfire will have pretty obvious consequences due to the fact that the engine in your GM car is running on only 3 cylinders. If the engine in your car has one or two misfiring cylinders, you'll have one or more of the following symptoms:

  1. The check engine light (CEL) will be on.
  2. One or more misfire codes (P0300-P0304) will be stored in your car's PCM memory.
    1. P0300 Random Cylinder Misfire.
    2. P0301 Cylinder #1 Misfire.
    3. P0302 Cylinder #2 Misfire.
    4. P0303 Cylinder #3 Misfire.
    5. P0304 Cylinder #4 Misfire.
  3. Sometimes, even tho’ the engine is suffering a bona-fide misfire, no misfire codes are registered and no check engine light (CEL) comes on.
  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.
  8. Will not pass the emissions tests.
  9. Bad gas mileage.

Although the misfire codes don't tell you what exactly is the cause of the misfire or rough idle condition... there is a way to find out exactly what is causing it.

One of the most important things you need to know, to successfully diagnose a misfire or rough idle condition, is what causes a misfire. Let's go to the next subheading and find out.

What Causes A Misfire Condition?

As I mentioned in the previous heading, each of the 4 cylinders of your Quad-4 engine needs 3 things to be able to produce power. In a nutshell, these 3 things are:

  1. Air
  2. Fuel
  3. Spark

It's when one of these three things is missing from the mix that the engine in your Quad-4 GM vehicle starts to misfire. Let's look into more specifics:

Ignition System: The ignition system is responsible for the production and delivery of Spark. The Ignition System is usually the culprit behind most misfires.

The usual suspects (that cause a misfire) are:

  1. BAD ignition coil (remember, each coil firs spark to 2 cylinders).
  2. BAD ignition coil cover.
  3. BAD spark plugs.
  4. Carbon tracks on the spark plug and spark plug boot.
  5. Oil dripping (from the valve cover) onto the spark plugs and spark plug boots.

Testing all of the ignition system components is not hard and it doesn't require expensive tools or expensive diagnostic equipment.

Fuel System: The fuel system is responsible for the delivery of fuel. If fuel is missing from any one specific engine cylinder, it will misfire.

fuel system problems could include some of the following:

  1. BAD fuel injectors.
  2. Broken fuel injector connector (this is a very, very common problem).
  3. Electrical short in the fuel injector wires that are keeping the fuel injector pulse signal from reaching the fuel injector.
    1. This is usually the result of human error and after a major mechanical repair where the wiring harness was damaged.
  4. BAD fuel injection computer not pulsing the fuel injector (this is a very rare condition, but it happens).
  5. BAD fuel pump.

Engine Mechanical Condition: The pistons and valves are the ones that draw air into the engine. Usually all cylinders wear out evenly... but every now and then, either thru' lack of maintenance or some mechanical problem, you'll have one or more wear out at an accelerated pace.

To make the long story short, those cylinders (with accelerated wear and tear) to produce a less than average compression value that will cause a misfire condition.

Other issues, that can not be overlooked are vacuum leaks.