Skip to main content

How to Diagnose Spark Plug Problems

I've worked in a variety of hands-on professions and love to write about topics that can save readers from needless expenses.

What Do Spark Plugs Do?

Spark plugs are a vital component to the internal combustion engine, and spark plug problems can stop you in your tracks . . . literally. They provide the burst of energy needed to ignite the fuel in your combustion chamber. The size of your engine, whether it has 4, 6, or 8 cylinders, will determine how many you have under your hood.

Each cylinder requires its own, and if one or more of your spark plugs stops working properly, you might experience a misfire in the corresponding cylinder. If you're not sure if your spark plugs are having problems, here are a few symptoms of problems:

  • Poor fuel economy
  • Rough idle
  • Stalling
  • Lack of power
  • Excess emissions

Diagnosing Spark Plug Problems

If you're experiencing one or more of the issues stated above, then you definitely might be having spark plug problems. But be aware that there are other conditions that can cause the same issues listed above, things like dirty fuel injectors, dirty air filters, or MAF sensor problems. So how can you tell which malfunctioning system is to blame for the problems you're experiencing? There are a few tricks.

1. Analyze your exhaust

You don't need to take your car into a smog center to do this. All you need to do is stand behind the vehicle while it's running at idle and get a scent of it to see if there's any smell of unburnt fuel in the air. If you have a faulty or fouled spark plug, then one of your cylinders isn't going to combust properly, leaving the fuel in the chamber unburnt. This same fuel will be pushed out of the tail pipe and should leave the scent of unburnt fuel in the air, which you should be able to detect. Please don't stick your nose in the tail pipe, and don't breathe the exhaust deeply into your lungs. Just try to get a scent of the exhaust.

2. Take your car to your local auto parts store

Your local NAPA or other auto parts store should be able to plug a diagnostic tool into your car which will be able to give them a code and a readout on your engine. If one or more of your cylinders is consistently misfiring, the diagnostic tool should be able to detect it and give a code for which cylinder is misfiring. This will be a very strong indication that you're having spark plug problems with that cylinder. Misfiring doesn't always indicate it, however, which you should be aware of. But one of the main causes of misfiring is a faulty spark plug.

Causes of a Spark Plug Going Bad

Spark plug problems can arise from a variety of things. Since they are continuously in use while an engine is running, they have a natural shelf life and can wear out. But certain conditions can cause them to wear out faster than usual or to go bad very quickly. Here are some of the main ways that they can go bad:

1. Oil in the combustion chamber

As vehicles get older and start to wear out, some of the oil that lubricates the engine can begin to enter the combustion chamber where the fuel/air mixture is ignited. A slow steady flow of oil into the combustion chamber is one of the leading causes of spark plug problems. It can cause the tip of the plug to get oily and dirty, resulting in buildup and damage to the plug, which will ultimately cause it to fail. If you have an older vehicle and you know it's burning oil, then the lifespan of your spark plugs will most likely be shorter than usual.

2. Improperly gapped spark plugs

As you can see from the picture of the spark plug above, there's a small gap in between the metal pieces on the tip. This gap needs to be precisely measured for the specific type of engine when a spark plug is installed. If the gap is made incorrectly, then extra stress might be placed on the tip, causing it to erode and wear out faster. Each one is gapped separately upon installation. So if one of them is slightly off, it can cause that single plug to go bad before the rest of the set.

3. Carbon buildup

Just like oil can cause a spark plug to go bad, so can carbon buildup. Instead of an oily look, sooty carbon deposits have a dry and black look to them. So if you're looking at your spark plug yourself, that can help make the determination as to what specific problem you might be having. Carbon buildup can be caused by a number of factors. A cracked distributor case, dirty fuel injectors, improper fuel/air mixture, and clogged air filter element, to name a few can cause carbon to build up on spark plugs and quickly cause problems.

4. Overheating

Overheating of the tip of the spark plug can cause it to wear out prematurely. This can arise from a number of different causes. If the timing of the engine is off and pre-ignition occurs, this can cause excess heat to build up in the combustion chamber. When an engine is running, the fuel/air mixture is first compressed before it's ignited.

If the mixture is ignited slightly early before it's fully compressed, this is known as pre-ignition. In addition to pre-ignition, if your engine's cooling system isn't functioning at peak performance and your engine tends to run hot, this can also cause overheating to your spark plugs and give them a shorter shelf life.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.