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Using Spark Plugs To Health Check Your Engine

Learn to read your spark plus, it's easy and they could be pointing out problems

Use Spark Plugs to Check Your Engine Health; It's easy when you know how, our guide to quickly find out if your plugs are pointing out problems

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Spark Plug Firing End Appearances And What They Mean

Ensuring your spark plugs are functioning correctly is an important part of keeping your engine in good health. But how can you tell the difference between a plug that's doing it's job, and one that's on it's last legs? Well, the appearance of the firing-end of a used spark plug directly reflects the condition of an engine, so Opie Oils and NGK have got together to show you the different appearances of the firing end of a spark plug and how to interpret what this can actually mean.

Normal Condition:

Normal Condition Spark Plug

If the firing end of the spark plug is brown or light grey, the engine condition can be judged to be good and the spark plug is functioning correctly.


Spark Plug Deposits

The gathering of different deposits on the firing end is influenced by oil leakage, fuel quality and engine operating period. The deposits can come from Carbon, Lead, Bromine, Calcium, Sulphur, Barium and Zinc.

Dry and Wet Fouling:

Spark Plug Dryfouling

Dry and Wet Fouling is another way the firing end can be damaged. If the insulation resistance between the centre electrode and the "Shell" is over 10M ohms the engine can start up normally, however if the insulation resistance drops to 0 the firing end is fouled by either wet or dry carbon.

Lead Fouling:

Spark Plug Lead Fouling

If your spark plug firing end looks yellowish brown on the insulator nose, this is found on spark plugs that have been damaged by lead. Also, this particular type of damage cannot be detected by a resistance tester at room temperature. Lead compounds combines at different temperatures; those formed at 370-420 degrees Celsius have the biggest influence on the resistance.


Spark Plug Overheating

If your spark plug has overheated, the insulator tip is glazed or glossy. Deposits which have gathered on the insulator tip have melted, and there is a chance that the insulator will have blistered.


Spark Plug Breakage

Breakage is normally caused by a thermal shock due to sudden heating or cooling – replace immediately!

Normal Life:

Spark Plug Normal Life

A worn spark Plug not only wastes fuel but also strains the whole ignition, this is because is requires a higher voltage. A worn spark plug can reduce the engine efficiency by reducing the fuel economy and increases the exhaust emissions. For your reference, The normal rate of gap growth is about 0.01 0.02mm/1,000 Km for four stroke engines and about 0.02 0.04mm/1,000 Km for two stroke engines.

Erosion, Corrosion, Oxidation:

Spark Plug Erosion

The electrodes have oxidized, and when the oxidation is heavy there will be green on the surface. The surfaces of the electrodes are also fretted and rough.

Abnormal Erosion:

Spark Plug Abnormal Erosion

An Abnormal Erosion is caused by corrosion, oxidation, or reaction with the lead. This results in abnormal gap Growth.

Lead Erosion:

Spark Plug Leaderosion

Lead Erosion is caused by the lead compounds in the gasoline which react chemically with the material of the electrodes (nickel alloy) at high temperatures. Crystals of nickel alloy fall off because of the lead compounds permeating and separating the grain boundary of the nickel alloy. Typical lead erosion causes the surface of the electrode to become thinner and the tip of the electrode looks like it has been chipped.


Spark Plug Melting

If the firing end is melted, this means it has over heated. Mostly, this will result in the electrode surface being rather lustrous and uneven. FYI, the melting point of nickel alloy is 1,200 – 1,300 degrees Celsius.


If you're wondering how often should you change your spark plugs, Opie Oils are sorry to say there is no single simple answer to this question. The best guide is the manufacturer's recommendation for your vehicle, as this particular service varies from manufacturer to manufacturer and brand to brand.

Looking for the right spark plugs?

Opie Oils has a simple lookup that can find the correct spark plugs for your vehicle in seconds

Spark Plug Lookup