How to: Reading Spark Plugs – Heat Ranges and Conditions

Have you ever pulled out your spark plugs, taken a look… and then discovering you really don’t know the condition of the plug? Unless you are documenting the mileage on each part you replace, most of us novice enthusiasts end up guessing the condition of the plug, and maybe even replacing spark plugs prematurely. Don’t waste your money folks. Reading your spark plugs ends up being quite an easy task. *Images have been referenced from and*

A Normal Spark plug heat range is indicated when:

The insulator nose is white or very light tan to a light, rusty brown. You should see little to no cement boil where the centre electrode protrudes through the insulator.

You’ve bought plugs that are too cold!!!:

The Insulator nose will be dark grey, perhaps even black. Steel plug shell ends will be covered with a dry, black, sotty deposit that you can easily wipe off.

Oh god, it’s too hot:

The spark plug insulator nose will be chalky white, possibly a satin like sheen. Unlike a plug gapped at the correct heat range, you will see excessive cement boil at the centre electrode protrusion. At the same time, the electrode may be ‘blue’ and rounded off at the edges.

Pre-ignition, YOU DON’T WANT THIS!!!:

Your insulator nose is blistered. The Centre electrode and side electrodes will be burned/charred, and has a molton like appearance. If this happens then you are most definitely going to look at a colder step of plugs.

Detonation is just a direction that spells major trouble:

The plug insulator will be fractured with tiny pepper like specks or even tiny beads of aluminum from your PISTONS. Excessive cement boil occurs at the centre electrode and there are specks on the plug shell end’s as well. If you haven’t totally destroyed your valve train yet, Retard your ignition and richen your fuel mixture.

Insulator Glazing:

Replacement of the plug is suggested with replacements belonging in the same heat range. If the glazing still occurs, try bumping up to another step colder. The plug will have a Shiny yellow , green, or tan insulator nose.

Ash fouled spark plugs needed to be cleaned or replaced with plugs of the same heat range. The plug will have a thick yellow, white or light brown deposit on the insulator, as well as the centre to side electrodes.


– Billy


About awdaltima

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One Response to How to: Reading Spark Plugs – Heat Ranges and Conditions

  1. Pingback: spark plugs; step colder? - Nissan Forums: Nissan Enthusiast Forum

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