How To Troubleshoot A Misfire (GM 4.8L, 5.3L, 6.0L)

Finding the exact cause of a misfire can sometimes be a challenge that can have you pulling out your hair (and spending money on parts that don't solve the problem). Why? Because anything from a bad COP Coil to low engine compression can cause a misfire.

Well, troubleshooting and solving a misfire doesn't have to be hard. Armed with the knowledge of what can cause a misfire, rough idle condition and a specific testing strategy, you can easily find out what component is the one that has failed.

In this article, I'll talk about the most common causes of a misfire and I'll show you a specific diagnostic strategy that'll help you track down and solve the issue.

You can find this tutorial in Spanish here: Cómo Diagnosticar y Resolver Una Falla en Cilindro (GM 4.8L, 5.3L, 6.0L) (at: autotecnico-online.com).

Symptoms Of A Misfire

The core symptom of a misfire is rough idle. This isn't the only symptom that your GM van, pickup or SUV might experience... here are some more that you might see:

1. Check engine light shining nice and bright and one or several of the following trouble codes stored in the PCM's memory:
   • P0300: Random Cylinder Misfire.
   • P0301: Cylinder #1 Misfire.
   • P0302: Cylinder #2 Misfire.
   • P0303: Cylinder #3 Misfire.
   • P0304: Cylinder #4 Misfire.
   • P0305: Cylinder #5 Misfire.
   • P0306: Cylinder #6 Misfire.
   • P0307: Cylinder #7 Misfire.
   • P0308: Cylinder #8 Misfire.
2. Really bad gas mileage.
3. Lack of Power when you accelerate your van, pickup or SUV.
4. Engine may start, run for a few minutes and stall.
5. Smell of raw gasoline coming out of the tail-pipe.

Basics Causes Of A Misfire Condition

A misfire will always be caused by a failed component in one of the three basic systems of your vehicle. These are:

1. The fuel system.
2. The ignition system.
3. The engine (piston, cylinder, and cylinder head valves).

Getting down to the bare essentials that the engine, in your vehicle, needs to start and stay running, the engine needs: Air, Fuel and spark.

When things are running optimally, each engine cylinder gets its full dose of air, fuel and spark. It's when one of these three (air, fuel or spark) are missing from the mix that the engine in your GM pickup, van or SUV starts to misbehave and misfire.

Now that we're aware of the core essentials that each engine cylinder needs to run at a 100% power output.. in the next sections I'll explore some of the specific components that fail and cause a misfire and misfire codes.

With And Without Misfire Codes

A misfire condition that sets a diagnostic trouble code (P0301, P0302, P0303, P0304, P0305, P0306, P0307, P0308 ) and thus illuminates the check engine light is one that is usually easy to diagnose and repair. Why? because the PCM is letting you know what specific engine cylinder is the one having the 'coughing fits'.

Knowing which specific cylinder is the one that's dead or missing, is a time saver! What sucks is that the diagnostic trouble code doesn't tell you what exactly has failed.

That's right, the misfire could be caused by a bad COP Coil (the usual culprit), or a bad fuel injector, or low compression in one or several cylinders.

The hard-to-diagnose misfires are usually the ones that set a P0300 Random Misfire Code or ones that don't set a diagnostic trouble code at all.

I don't mean to give you the idea that it's all a hopeless matter! -Because the cool thing is that no matter if you have misfire codes or No Codes at all, the core cause of the misfire is a lack of spark, or fuel or air (think engine mechanical problem such as: vacuum leaks, low compression, etc).

Let's turn the page and we'll take a look at what you'll need to do to pinpoint and resolve the issue causing the misfire.

Ignition System: Misfire Troubleshooting

The most logical starting point, when troubleshooting a misfire on your 4.8L, 5.3L or 6.0L pickup, van or SUV is testing the COP coils for spark.

Why? Because a failed COP coil is usually the reason behind the misfire in the majority of cases. Here are my specific recommendations:

1. Check for misfire codes with a scan tool first.
   • The misfire codes (other than a P0300) identify the engine cylinder that's missing or dead.
   • If no misfire trouble codes are present or you have a P0300, then a cylinder balance test is recommended to find the misfiring cylinder.
     • The cylinder balance test should be done by unplugging the electrical connector of the fuel injector or the COP Coil.
2. Remove and Inspect:.
   • Spark Plugs for:
     • Cracks.
     • Engine oil (from a leaking valve cover gasket).
     • Fuel fouling.
     • Center electrode capped/blocked with carbon.
     • Check for carbon tracks on the ceramic insulator.
   • COP Coils Spark Plug Wires for:
     • Cracks or burns on the spark plug wire's rubber boot.
     • Carbon tracks on the inside of the rubber boot.
3. Test for Spark.
   • Use a dedicated spark tester.
     • Doing a spark test with anything other than a spark tester (like pulling the COP coil or spark plug wire from the spark plug) can and will damage the ignition coil.
     • Using a timing light will only have you wasting your time and money since this is the MOST INACCURATE way to test for spark.