Diagnosing a misfiring Quad 4 engine in all of the models that use this engine can be a challenge. This is a case study of one Pontiac Grand Am that was misfiring with only a P0300 Random misfire code stored in the vehicle's computer. This info supplements the article How To Test The Ignition Coils On The 2.4L Quad 4 Engine and will teach you some new tests not covered in that article.
Although this case study involves a 1996 Pontiac Grand Am, you can apply this info to any GM make and model that uses this type of ignition system.
Contents of this tutorial:
You can find this tutorial in Spanish here: Diagnóstico De Falla de Encendido GM 2.4L Quad 4 (Estudio De Caso) (at: autotecnico-online.com).
Diagnostic Starts Here
The very first thing I did was to check the codes lighting up the check engine light (CEL) on the instrument cluster. And one of the two codes stored was the P0300 Random misfire code and the other was an oxygen sensor rich code. As you'll probably agree, not much to go on since any number of conditions or bad components can cause these codes.
I could see that the engine was indeed misfiring and as I drove it into my service area (at the auto repair shop where I work), I also could feel it. OK, once I got it in, the very first thing I did was to find out if the misfire was being caused by a bad ignition coil. I followed the steps in TEST 1: Testing For Spark (of the tutorial: How To Test The Ignition Coils On The 2.4L Quad 4 Engine).
Just to paraphrase TEST 1, the very first thing that has to be done is a spark test. The idea is to test the ignition coils to see if they're bad by attaching an HEI spark tester to each spark plug boot at a time to see if they spark or not.
If one or several do not spark, then there's a possibility that an ignition coil could be bad, and of course there are several other tests to perform to further verify this.
Now in the case of this car (the Grand Am), all of the spark plug boots fired off spark when I did TEST 1. Not only that, but there were no carbon tracks on the spark plug boots, nor on the spark plugs, and they weren't swimming in engine oil either. The coil cover had no cracks or carbon tracks either. As a matter-o'-fact, they all looked new (tho' the customer hadn't mentioned he had already replaced all of these components already).
I was starting to think that the misfire cause didn't lie in the ignition system, but as you'll see, the problem did lie in the ignition system, since what solved the problem in this particular car was replacing one of the two ignition coils that was bad and this is how I found out:
Cylinder Balance Test
In cases like this, when the PCM (Powertrain Control Module=Fuel Injection Computer) doesn't provide a specific misfire code to let you know what cylinder is the one suffering the misfire (this happens a lot), you have to perform a cylinder balance test to find out. Knowing what specific cylinder is the one that is misfiring is critical to successfully diagnose the problem and to replace the correct part or parts.
A cylinder balance test is not that hard to perform on your own and does not require any expensive tools either. On the Quad 4 engine, in a nutshell all that's required is to have the engine running and to disconnect and reconnect one fuel injector at a time to see if unplugging the fuel injector affects engine idle.
And this is exactly what I did. The cylinder balance test let me know that it was cylinder #4 that was dead. How? Well, when I unplugged the cylinder #4 fuel injector, nothing happened to engine idle. Now, when I unplugged the others (#3 thru' #1) one at a time, the engine idle became worse/rougher because these were contributing to engine power. But not so on the #4 fuel injector.
The next step is to find out if the cylinder is dead (not contributing to overall engine power) due to one of these three reasons: 1) engine mechanical problem in cylinder #4, or 2) a bad fuel injector or its circuits or 3) some component of the ignition system. So, this required another round of simple tests, in the next page I'll go into more detail.
