Why do spark plugs have white deposits and how to fix them?

Spark plugs have a crucial role in proper engine operation. They are consumables and will need replacement whenever they wear out.

But spark plugs can help detect other problems besides their wear and tear, and the colors they may have on the body might indicate a potential engine problem.

Spark plugs operate in high-temperature environments, leading to the formation of thin light gray, yellowish or brown deposits on them. 

These colors are normal and they are given by fuel impurities and iron oxide, which are formed when oxygen is applied to the steel body. These colors will change in the event of a malfunction.

If you see white build-ups on the spark plugs, this might be a problem in the power supply or ignition system, or the wrong fuel is being used in the vehicle.

You should not worry too much if you see very weak white build-ups on the spark plugs and you have an LPG system installed in your car, even if they are glossy or thick.

Gas does not have special additives, such as gasoline or diesel. Its combustion temperature is slightly higher, and a small white carbon deposit will form on the body.

Therefore, if the engine runs smoothly, there is no excessive fuel consumption and antifreeze leakage, and there are no errors in the ECU, then there is no reason for concern.

Why do white build-ups form on the spark plugs?

White build-ups on spark plugs are often a cause of overheating because of a bad ignition process caused by a suboptimal fuel-air ratio or the ignition is not present at all.

Being exposed to high temperatures, the dark carbon deposits burn out, while the more persistent light-colored ones remain.

Causes and solutions of white spark plugs

1. Unstable air-fuel mixture

In this case, the engine will not work with optimal parameters. You should check the intake tightness, injectors, and some sensors like the camshaft position sensor, crankshaft sensor, and MAP sensor.

2. Spark plugs are worn.

If these parts are failing and you also use low-quality fuel then this can lead to white deposits on the spark plugs. In this case, you will the that the engine’s performance is significantly reduced and its cycle disrupted.

You can try to clean the spark plugs by adding quality fuel and extra additives, but a better solution would be to change the spark plugs

3. The car is using low-quality fuel.

If you are using fuel with bad additives or counterfeit fuel expect to see white build-ups on spark plugs. This aspect will cause poor engine operation, and also engine parts to wear faster, especially the oxygen sensor and the catalytic converter which will fail much faster.

Like in the case above, refilling with high-quality fuel and replacing the spark plugs may resolve the problem.

Glossy white deposits on spark plugs

Depending on the problems that can occur the deposits on the spark plugs have various forms. A glossy white deposit usually appears on the spark plug’s ground electrode.

This will negatively affect the engine operation in the long run, and it can indicate some problems that need attention.

The causes of glossy deposits on the spark plugs are:

  • Lean fuel mixture;
  • Intake air leaks;
  • Clogged injector nozzles;
  • Wrong spark plugs;
  • Fuel octane is higher or lower than required

These causes will lead to cylinders and valves overheating, and increased piston and spark plug wear. The engine oil will lose its protecting properties faster.

Also, the engine’s power will be reduced and it will receive less fuel than needed causing a lean mixture, a bad ignition, and the appearance of detonation (engine knocks).

You should check the intake system (pipes, intake manifold gaskets, nozzle seals) for leaks, and the injectors. Adjust the ignition system and also you might need to check the timing belt installation. Drain low-quality fuel and add high-quality fuel with the proper octane number.

Last but not least, replace all the spark plugs with new ones according to the requirements given by the manufacturer.

Thick, rough white soot deposits on the spark plugs

If you see this kind of deposit, then other substances such as coolant or oil could be present in the combustion chamber.

In this case, you will need to check the engine as soon as possible, the reasons for this problem are, usually, valve seals and cylinder head gasket that will need to be replaced.

Also, rough white soot on the spark plugs, as in the case of glossy build-ups, is a result of improper mixture formation, untimely spark, and additives and impurities in the fuel.

Why does thick white soot appear on the spark plugs?

Above I have listed the common reasons why you can see the rough white soot on the spark plugs, but there are also other reasons:

Fixing this problem can be challenging because there are many reasons why this can happen. Go to an authorized service and make all the possible diagnoses to find the right cause.

How to check spark plugs for white deposits

Whether the spark plugs have black deposits or white, knowing the right cause will prevent serious problems. For good maintenance, will need to perform a regular check of the spark plugs. For that, you will need:

  • spark plug wrench;
  • flashlight to look closely at the color of the spark plugs, especially in case of low light or total lack of it;
  • Wipes to clean the spark plug wells before removing them, and cover them for the test.

The procedure is easy and will not take that long. The only difference is that in some models it will be necessary to first remove the high-voltage wires from the plugs, and other individual coils, which are fastened with screws, will require an appropriate socket wrench or head with a screwdriver.

Take them off one at a time or mark the spark plug wires to not confuse them.

How to remove white deposits from spark plugs

Remember that you should clean the spark plugs after you have initially taken care of the problem that caused the soot in the first place. If there are very few deposits, the spark plugs can be cleaned and reused, but if the soot or build-ups are thick and in a larger quantity, they will need replacement.

There are two ways you can clean white accumulations on spark plugs, the first method, and the easiest, is you need the following things:

  • A coarse metal rust removal brush;
  • Fine sandpaper, P 240 and above

Carefully clean the plaque in the gap between the electrodes with fine sandpaper, folding it in half. When doing this, be cautious not to scratch the spark plugs. If you are using iridium spark plugs, don’t try to clean them this way.

Another method you should try is to chemically clean the spark plugs. This is the preferred way to clean them without damaging the electrodes. It works very well for more expensive spark plugs. For this, you will need

  • Solvents like gasoline, kerosene, acetone;
  • Orthophosphoric acid solution;
  • Vinegar or ammonium acetate solution of 20%
  • Sanitary plaque removers.

You can start by degreasing the spark plugs with a solvent, after that add the spark plugs with the working part (with the electrode) in the cleaning agent.

You can add the cleaning product to a bowl and put the plugs there. After that leave them for several hours controlling the rate of deposit removal and rinse the spark plugs again with solvent.

After removing the white build-ups, the spark plugs can be dried and installed in the engine. To speed up chemical reactions, non-flammable liquids can be heated, but not brought to boiling temperatures. Also, don’t forget safety, use gloves.

Sometimes, the soot does not completely come off when using this method, but it is enough to make it easier for a cloth to do the trick.


The presence of white soot deposits on spark plugs can have many causes. If the source of these build-ups is not identified and resolved, then the engine will suffer in the long run.

I hope the above explanations gave you an idea of why these build-ups are formed and how you can deal with this issue.

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