• spark plugs

Why do my spark plugs keep getting dirty?

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Updated 10 Oct 2019

Zak Zuiderduin

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First, let’s go over what the spark plug actually does.

Their job is to ignite the air-fuel mixture in the combustion chamber, giving energy to the pistons, which move the crankshaft, which ultimately gives rotational energy to drive the wheels.

The spark plug is where it all begins.


When the spark plugs get dirty, they cease to operate correctly.

We refer to this as ‘fouling’.

If your spark plugs are fouling, they will cause the engine to run rough, lack power, use more fuel, or even not run at all.



When a plug is carbon fouled, you will see dry, soft, black carbon deposits form in large quantities on the insulator nose and electrodes.

As carbon accumulates, the insulation between the centre and ground electrodes deteriorates, an electrical leakage path is formed by the carbon, and misfire results.

Causes of carbon fouling include rich fuel mixture, clogged air filter, prolonged low-speed driving or idling, faulty ignition system, retarded ignition timing and spark plug heat rating is too cold.


With oil fouling you will see wet, black, oily, carbon deposits form on the insulator nose and electrodes.

This will also result in an engine misfire.

Causes of oil fouling include excessive oil entering the combustion chamber.

This can be caused by the oil level being too high, through worn piston rings, or worn valve guides.

DEPOSITS (Ash Formation):

Heavy ash deposits build-up on the firing end of the spark plug and eventually cause a misfire.

In some circumstances these deposits can reach temperatures which might lead to pre-ignition.

Ash formation is mainly produced from the burning of oil, fuel additives, poor quality fuel and engine condition.


Overheated spark plugs have a white glazed or glossy insulator.

Small black deposits accumulate on the insulator nose and the electrodes are prematurely worn.

Causes of overheating include over-advanced ignition timing, lean fuel mixture, insufficient fuel octane rating, excessive deposits accumulated in combustion chamber, manifold air leak, insufficient cooling and lubricating, or the spark plug heat range being too high.

If your spark plugs get dirty, your vehicle won’t run efficiently.

This will increase the cost to run the vehicle and can cause permanent damage.

A professional mechanic on AutoGuru can diagnose these faults by inspecting your spark plugs and THEN determine the best course of action to rectify the problem.

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Written By

Zak Zuiderduin

Zak has been working in the auto industry for 40 years.

For the past decade, he has been in the education business, training apprentices and preparing them for a career in the light vehicle and auto electrical trades.