Spark Plug Diagnostics

As the spark plug is vital to your engines running and performance, it's advisable to occasionally check their condition. Not only can you discover wear and tear of your spark plug, but with these easy to follow pictures, you can also learn a lot about the state of the engine itself.

Before you check your spark plugs, we do recommend the following to help make an accurate diagnosis: Drive your vehicle for a distance of preferably over 10 miles. While doing so, make sure you vary the revs in the mid performance range and avoid extended idling before turning the engine off.

Normal

Insulator nose from grey-white / grey-yellow to russet colour. Engine in order, heat range correctly selected.

Mixture and ignition adjustment perfect, no misfires, cold start equipment functional. No residues of leaded fuel additives or alloy constituents from engine oil. No thermal overload.

Sooted

Insulator nose, electrodes and spark plug housing covered in velvety matt black soot.

Cause: Faulty mixture adjustment (carburettor injection), mixture too rich, air filter badly contaminated, automatic starter not in order or choke out too long, predominantly short trips, spark plug too cold, heat range code number too long.

Effect: Misfires, poor cold start characteristics.

Remedy: Adjust the mixture and starter correctly, check air filter.

Oil Contamination

Insulator nose, electrodes and spark plug housing covered in oily soot or oil carbon.

Cause: Too much oil in the combustion chamber. Oil level too high, badly worn piston rings, cylinders and valve bushings. In two-stroke spark-ignition engines, too much oil in the mixture.

Effect: Misfires, poor cold start characteristics.

Remedy: Overhaul engine, correct oil-fuel mixture, new spark plugs.

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These pages provided for information only, the authors accept no responsibility for any loss, whether financial or otherwise incurred through the use of the information on these pages. By reading these pages you accept this disclaimer. All content © 2001 Steven Crook-Dawkins and Richard Hardesty.