One thing I see quite a bit these days, is vehicles coming into the shop running badly, misfiring, check engine light code P0300 and getting very poor fuel economy. Usually upon a test drive, I’ll find it stumbling on acceleration. Back at the shop, while checking for trouble codes there will most times in fact be multiple codes for misfires. This is often due to problems with ignition system, particularly the spark plugs failing.
What is happening is called ‘carbon tracking’. This is where there can be a small crack in the porcelain of the spark plug and it allows the spark to arc out and not make the full journey to the end of the electrode in the cylinder. Once this occurs, you will get a random P0300 and it only gets worse over time. Accelerating from a stop, or trying to pick up speed on the highway to make a pass are when the carbon tracking misfires will happen. The more and more it does, it will build up an actual track of carbon on the outside of the spark plug, hence the name ‘carbon tracking’. If left without fixing, it can damage the ignition wire as well.
This has gotten so bad on some vehicles, that there has actually been factory recalls to check for this problem and replace the affected plugs and wires.
As you can see in the picture below, I have three spark plugs, each with traces of carbon tracking. The one on the right is by far the worst and has been driven a very long time. The spark plug in the middle has signs of this just starting, but it is still a problem. Finally, the spark plug on the left is about mid-way between just beginning and completely carbon tracked.
Any of these stages would require spark plug replacement. I would usually recommend replacing the ignition wires as well, just to be on the safe side. This will always cure this type of P0300 misfire and stumble upon acceleration.
If you are experiencing a problem like this, it would be a good idea to get the ignition system checked out for signs of carbon tracking. This is a common failure, and luckily not an extremely expensive repair!