We've all seen them: P0300, P0301, P0302, P0303, P0304, P0305, P0305, P0307, and P0308. The dreaded misfire codes! Sometimes they're a breeze to troubleshoot and solve, and sometimes they can have you pulling your hair out!
So many things can cause an ignition misfire, in this case study I'll show some effective but simple tests to diagnose misfire codes and to look for one of the most overlooked causes of ignition misfire: carbon tracks.
Before I start ‘spinning my yarn’, the following tutorials might also come in handy:
- How To Test The Coil Pack (Chrysler 2.0L, 2.4L).
- How To Test Misfire Codes (Chrysler 2.0L, 2.4L) (at: troubleshootmyvehicle.com).
- How To Test A Fuel Injector (Chrysler 2.0L, 2.4L) (at: troubleshootmyvehicle.com).
- How To Test The Engine Compression (Chrysler 2.0L, 2.4L) (at: troubleshootmyvehicle.com).
- How To Test The Crank Sensor (2.0L, 2.4L) (at: troubleshootmyvehicle.com).
Puedes encontrar este tutorial en Español aquí: Trazas de Carbón Provocan Fallas de Encendido (en: autotecnico-online.com).
The Customer's Complaint
The customer's complaint was that his daughter's 2003 Dodge Neon had the ‘check engine’ light on and the engine was missing, especially when he gave it gas.
One of the things I love about the place where I work is that I get to interact with the customer quite a bit. This is important because I can directly ask the customer a lot of different questions about their vehicle that will speed up my diagnosis and in turn save them time and money.
Usually my very first question is: What have you replaced already? On this particular vehicle, the customer responded that he had already replaced the spark plugs... twice.
In this tough economy, I'm seeing more and more folks coming thru' the shop doors that are working on their own cars. Bless their hearts. There's nothing wrong with trying to save a buck. When they get stuck, thankfully they come to us.
I asked him what made him think it was the spark plugs. He mentioned that he had gone to AutoZone to take advantage of their infamous ‘free diagnostic check’. Got the car scanned and what came up on their scan tool was some misfire codes. The friendly folks in orange there suggested that he replace the spark plugs, which he did. And as we now know, to no avail.
As I heard the story, I started thinking about several possibilities such as: a worn cylinder not contributing to the overall engine performance, a bad fuel injector, or a bad ignition coil. Thankfully, it wasn't something serious at all.
First Step In Ignition Misfire Diagnosis: Check For Codes
After the customer and owner of the shop finally walked away and the car was in my service area, I could finally start. But start where?
The first thing that has to be done is to check for trouble codes with a scan tool.
Out came the scan tool and after connecting it, sure enough: two misfire codes. A P0302 Cylinder #2 misfire and a P0300 Random misfire code. Now I knew what was lighting up the ‘check engine’ light!
This in itself doesn't tell me anything specific. A scan tool will never tell you specifically what the source of the problem is. Now, don't get me wrong -a scan tool is a must have tool. But it is just a tool.
Using a simple analogy. An air ratchet is a great tool that can help you to remove the cylinder head from a block. But if you don't know how to remove and install the cylinder head, the ratchet would be useless.
The same applies to a scan tool. To successfully use a scan tool you must have some knowledge of how the system works (theory) that you're testing. You must know how to test the components of that system or systems that the scan tool indicates as having or causing the problem. The bright side is that this site will be one of the many sources you can use to gain some of that knowledge.
Anyway, getting back to the Neon with the misfire -as you can see in the photo, this car has an ignition coil pack that feeds spark to the different cylinders from one assembly, as opposed to having an ignition coil sitting on top of each individual cylinder or a distributor. This ignition coil pack sits on top of the valve cover.