Spark plugs spend their entire functional life inside the combustion chambers of your engine. There, it’s exposed to fuel, air, and constant explosions that it triggers by causing a spark (hence the name). Over time, though, spark plugs can experience carbon fouling and become less useful.
Carbon fouling happens when the car’s air and fuel mixture is too rich, which means the proportion of fuel is too high. It results in incomplete combustion, leaving behind a carbon buildup on the spark plug’s firing end. Simply put, carbon fouling is a symptom of a fuel problem, not a spark plug problem. You may use sandblasting to remove that buildup of carbon, but it does not solve the root of the problem.
In this article, we’re going to take a deep dive into the issue of carbon fouling on your spark plugs. We’ll explore what it is, what causes it, and what symptoms to look out for to know if your plugs are suffering from carbon fouling. Lastly, we’ll look at how it’s typically fixed and how you can prevent it from happening again in the future.
Let’s get started.
What Is Carbon Fouling?
By definition, 'fouling' is when something unwanted accumulates on a hard surface. In the context of carbon fouling on spark plugs, it refers to excessive fuel carbon that accumulates on the spark plug electrode. This buildup has gotten to the point where the plug can no longer generate the sparks needed for efficient combustion.
If you remove the individual spark plugs and inspect them up close, what you’ll see is a grey or black matter all over the spark plug’s electrode and porcelain. That matte coating is the carbon buildup.
Other Types of Fouling on Spark Plugs
Here’s the thing: there are also a couple of other ways that your spark plugs can get fouled besides carbon from fuel. They are:
- Engine oil fouling – Unlike carbon fouling, this will give your spark plugs a somewhat inconsistent, shiny black finish.
- Coolant fouling – Caused by leaking gaskets that allow coolant to leak into the combustion chamber. This type of fouling usually looks grey and ashy but could also differ slightly depending on the type of coolant you use.
What Causes Carbon Fouling on a Spark Plug?
Carbon fouling on a spark plug is caused by a problem with the fuel that you use. Often, we describe it as a fuel mixture that's too rich, which means that the ratio of fuel to air is far too high.
That can cause combustion problems, resulting in excess carbon that sticks very quickly to the hot parts of the spark plugs.
Again, we must remember that carbon fouling (or any other type of fouling, for that matter) is only a symptom and not the root problem. So, if you know that you're experiencing carbon fouling, you must troubleshoot and resolve whatever issue causes it.
If not, any new spark plugs that you place in your engine will just get fouled up all over again.
Other Less-Known Causes of Carbon Fouling
Whenever you read or talk about the causes of carbon fouling, the first answer you’ll always get is that the fuel mixture is far too rich. That’s still true, but here are a few other possible reasons behind carbon fouling:
- Driving short distances most of the time – When you do this too much, your engine never has enough time to warm up and not perform the combustion process optimally.
- Problematic fuel injectors – The injectors may be clogged, stuck open, or there’s an injection timing delay.
- Wrong type of spark plugs – Plugs with a heat range that’s too high for your car could also cause this problem. For more information about spark plug heat ranges, check out this article here.
What Are the Signs of Carbon-Fouled Spark Plugs?
The most effective way to diagnose your spark plugs is to take them out and inspect them with your own two eyes. As mentioned before, carbon fouling will appear as a grey or matte black finish on the spark plug’s electrode and porcelain.
Still, there are some other tell-tale performance symptoms that you can look out for before you choose to do that. They include:
- Poor starting
- Misfiring – carbon buildups can conduct electricity and cause a spark, even when the spark plug shouldn't
- Acceleration problems
Sure, those symptoms can point to a long list of different problems under the hood. However, it might help to inspect your spark plugs first and rule out carbon fouling.
Can You Clean a Carbon-Fouled Spark Plug?
Yes, it’s possible to clean a carbon-fouled spark plug instead of replacing it entirely. Some of the methods used are:
Sandblasting – Using a machine that shoots sand out with a jet of air to scrape the carbon off the spark plugs
Burning with a butane torch – Some DIY repairers have also shared that it’s possible to burn off the excess carbon with a butane torch
Using wire brush or sandpaper – To scrape off the carbon manually
Applying spark plug cleaner – Some products on the market can help you keep your spark plugs clean. These generally don’t cost too much, and you can find them at your preferred auto parts store.
How Do You Prevent Carbon Fouling?
The most important thing you need to remember is that carbon fouling is a symptom and not the root cause. So, suppose it has already happened, but you haven’t solved the main issue; it only means that you will experience it again and again until you solve the main issue.
Here are some preventative measures you can take to protect your spark plugs from carbon fouling:
- Warm up your engine – If you typically drive only short distances, give your engine some time to warm up hitting the road. When the engine works at optimal temperatures, it helps in preventing carbon buildup at your spark plugs.
- Keep your fuel injectors in good condition – As mentioned earlier, a clogged injector or one that’s stuck open could be the culprit, so make sure to inspect it regularly.
- Use the correct spark plug heat range – For more information on this, you can check out our previous article about spark plug heat ranges.
Besides that, you may consult your mechanic, the seller you're buying your plugs from, or even your car manufacturer. Your vehicle’s User Manual might also have recommendations on what spark plugs you should use.
Be a Smart Car Owner! Learn from CarPartAU!
To learn more about issues like carbon fouling and spark plugs, head on over to the blog at Carpart.com.au. There, you’ll have access to plenty of free articles teaching you about auto maintenance to help you become a better-informed car owner!
By Ray Hasbollah