How To Read Spark Plugs – Everything You Need To Know

The spark plugs play essential roles in every internal combustion engine. You can invest in fancy head cylinders, turbo or superchargers, engine management, or cold intakes. Still, if you’re not paying enough attention to the spark plugs, you’re letting engine horsepower slip through your fingers.

In all honesty, the results of every engine management rely on the tool used to calculate the air-fuel mixture. Spark plugs are the only true way to understand what’s happening in the combustion chamber. Here, we’ll explain how to read spark plugs so you can tell when they’re bad or if something is wrong with the combustion chamber.

how to read spark plugs for tuning

How to read spark plugs step by step

Damaged or fouled spark plugs can lead to issues like poor gas mileage, poor acceleration, hard starting, and engine misfire. As explained earlier, reading your spark plugs will reveal what’s happening in the combustion chamber. You’ll likely notice one of the following when reading spark plugs.

  • Wet spark plugs
  • Oil on spark plug threads
  • Normal
  • Fouled
  • Worn electrodes
  • Broken electrodes
  • Burned.

Whether you’re searching for how to read spark plugs for tuning, how to read spark plugs for rich or lean conditions, or for any purpose, read to the end, when reading spark plugs, you have to locate the spark plugs and remove the ignition lead cables or the ignition coils. After that, grab the plug spanner and unscrew the plugs from the plug wells.

You can remove the spark plugs one at a time to avoid misplacing the lead cables in the wrong cylinder. Also, you can remove all the plugs at once if you have ignition coils. However, the latter may confuse you to mistake the spark plug numbers. Therefore, it is essential to remove and read the plugs one at a time.

Once you remove any spark plug, look at the plug to determine what’s going on in the combustion chamber. You’ll likely find one or more of the following signs at any given time.

Normal reading

If the spark plug is still okay, it’ll have a grayish-tan or brown deposit on the electrode. The plugs are okay if there’s nothing else on them. All you need to do is to clean the spark plugs and reinstall them.

Fouled spark plugs

A carbon-fouled spark plug will have a dry, black soot on the insulator tip and the electrodes. This can emanate from an engine running rich, clogged air filters, idling the car for too long, or frequently driving at low speeds. An experienced mechanic will track and rectify the root cause or advice you on what to do.

Sometimes, all you need to fix the issues is to clean the air filters or to do more long-distance driving. In other cases, getting hotter spark plugs could be all you need.

Oily spark plugs

A wet spark plug or oil-fouled spark plug indicates there’s an oil leakage into the cylinders. The oil could leak from cracked piston rings or bristled valve cover seals. You’ll see oil on the spark plug walls if a bristled, dry, or worn valve cover seal is the root culprit. You must track and rectify the root cause of the problem to fix the underlying issues.

Wet spark plugs

The two most prevalent causes of wet spark plugs are engine flooding and faulty fuel injectors. Engine flooding happens when you crack the engine several times without starting it up. The injectors will dump gas in the combustion chambers when you’re trying to start the engine. You can clean or let the fuel dry on its own.

If one or more spark plugs keep getting wet, you likely have a defective fuel injector sending too much fuel to the combustion chamber. The only solution is to identify the faulty injectors and replace them.

Burned spark plugs

White deposits, melted electrodes, or blisters on the insulator tips show you have burnt spark plugs. Burnt spark plugs simply mean the plugs are running too hot. The probable causes are lean running conditions, incorrect ignition timing, incorrect spark plug heat range, engine overheating, or loose spark plugs. You need to rectify the leading cause and replace the old plugs.

Worn electrode

A worn electrode shows the plugs are long gone. Even if the plugs are working fine, you need to replace them with newer ones.

Broken electrodes

A flattened or broken electrode shows that the engine is running with the wrong spark plugs. If you install longer spark plugs in your engine, they’ll cause catastrophic damage to the engine. Similarly, if you install shorter plugs, they’ll cause poor gas mileage or foul the plugs.

Therefore, it is essential to check your owner’s booklet and ensure you’re using the right set of plugs.

So, whether you are asking how to read spark plugs on methanol, how to read spark plugs on e85 or NGK, or how to read spark plugs, this is all you need.

Learn More: Good Spark Plug vs Bad Spark Plug—How to Identify Them

how to read spark plugs numbers

Frequently Asked Questions – FAQs

How do you read a lean or rich spark plug?

Most mechanics and DIYers do not know you can tell when the engine is running hot or lean by reading the spark plugs. If an engine is too rich, the spark plugs will look black or glossy black, depending on how rich the engine is. In contrast, the spark plugs will be greyish-white if the engine runs lean.

How do you read the timing mark on a spark plug?

One can tell if the timing is retarded, advanced, or normal. The spark plug electrode should burn evenly from the tip to the end. If the burning covers the electrode to the thread areas, the engine is likely 2 to 4 degrees advanced. And if the electrode burns evenly and stops halfway, the timing is likely 2 to 4 degrees retarded.

Here’s a visual presentation for better clarification if you don’t have the time to watch the videos to the end, jump to the ‘timing section’ on the timestamp.

What do the numbers on spark plugs mean?

Have you ever wondered what the numbers or alphanumeric letters on spark plugs represent? Firstly, the numbers on spark plugs represent the heat range of that plug. Each spark plug manufacturer indicates the heat rating of their plugs with numbers.

Lower numbers indicate the plug has a hotter heat range than the ones with a higher number. For instance, a YR5GP and YR7GP spark plug may come from NGK, but the YR5GP has a higher heat rate than the latter.

What color is a good spark on a spark plug?

When reading spark plugs, the colors determine whether the plugs are functioning well or not. A good spark plug should have a light brown or tan color on the tip of the electrode, indicating that it’s functioning optimally.

Any other color or mixture of colors shows there’s something wrong. However, it doesn’t necessarily mean the spark plug is bad. For instance, an oily, fouled, or wet spark plug means there’s oil leakage or you have faulty fuel injectors.

What color is a weak spark?

The spark color determines the strength of spark plugs. For example, a yellow or red spark is weak and may not even spark in the cylinders. On the contrary, a light blue or white spark is very strong and needs electric juice to travel across the plug gaps even under pressure in the combustion chamber.

How do you tell if an engine is a rich or lean burn?

When you hear lean or rich burn, it refers to how the engine burns an air-fuel mixture in the combustion chamber. Rich burn means there’s too much fuel or too little air in the chambers during combustion. Lean burn means too much air or too little gas in the chamber during combustion.

The spark plug electrode will be wet if the engine has a rich burn. And if the engine is running lean, the plugs will be burnt. However, it’s important to note that other issues can cause wet and burnt spark plugs.

What happens if I use a hotter spark plug?

Always stick with the manufacturer’s recommendations. The owner’s booklet is the manufacturer in print. Always follow the instructions and recommendations in that booklet.

Using a hotter spark plug can lead to overheating, reduction in fuel economy, pre-ignition, and accelerated wear on the electrodes. You don’t want to have pre-ignition because it can cause fatal damage to internal engine components.

What happens if you run a colder spark plug?

Using colder or hotter spark plugs cannot increase horsepower. Instead, it can cause multiple issues to the engine. If you use colder spark plugs than the recommended ones, they may not burn off carbon deposits properly. This will invariably cause plug fouling. Spark plug fouling will lead to engine stalling, rough idling, stuttering, and misfiring.

Final Words

How to read spark plugs is one of the basic skills every DIYer should know. Thankfully, this article has explained how to read spark plugs for tuning, timing, and numbers in a simplified way.

If you have been reading to this point, you can tell when your engine is running lean or rich, when the spark plugs are bad, or when something is wrong. We also offered recommendations on how to rectify the problem (if any). Follow the recommendations or have a mechanic track and fix the leading culprit.

Osuagwu Solomon

Osuagwu Solomon is a certified mechanic with over a decade of experience in the mechanic garage, and he has over five years of experience in the writing industry. He started writing automotive articles to share his garage experience with car enthusiasts and armature mechanics. If he is not in the garage fixing challenging mechanical problems, he is writing automotive repair guides, buyer’s guides, and car and tools comparisons.

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