Read Your Spark Plugs Like a Tuning Storybook

By March 25, 2015Engine
How to Read Spark Plugs

Spark plugs wear their life story on the outside and being able to read their appearance can be a valuable tuning aid. Just by looking at the insulator firing nose color, you can tell a great deal about the engine’s overall operating condition and troubleshoot problems ranging from poor gas mileage, loss of performance, idle roughness, hesitation, hard starting to even no start at all.

Here are some of the most common spark plug conditions:

Normal Spark Plug


Brown to grayish tan color with minimal electrode erosion indicates the plug is in the correct heat range. Combustion deposits are slight and not enough to cause any detrimental effect on engine performance.

Spark Plug with Mechanical Damage

Mechanical Damage

May be caused by a foreign object that accidentally entered the combustion chamber. It may also be due to improper reach spark plugs where the piston collides with the firing end.

Oil Fouled Spark Plug

Oil Fouled

Too much oil is entering the combustion chamber. It’s often caused by badly worn piston rings or cylinder walls. Oil may also be pulled into the chamber because of excessive clearance in the valve stem guides. If the PCV valve is plugged or inoperative it can cause a build-up of crankcase pressure which can force oild and oil vapors past the rings and valve guides into the combustion chamber.

Overheated Spark Plug


Characterized by a clean white insulator firing tip and/or excessive electrode erosion. This is typically caused by over advanced ignition, timing, poor engine cooling system efficiency (scale, stoppages, low level), a very lean air/fuel mixture, or a leaking intake manifold.

Glazed Spark Plug

Insulator Glazing

Glazing appears as a yellowish, varnish-like color. It means the spark plug temperatures have risen suddenly during a hard, fast acceleration. As a result, normal combustion deposits don’t have an opportunity to “fluff-of” as they normally do. Instead, they melt to form a conductive coating and leads to misfiring at high speeds.

Pre-ignition Spark Plug


Melted electrodes and a white insulator – although it may appear dirty on the surface due to one or a combination of engine conditions. It can happen from flowing combustion chamber deposits, hot spots in the combustion chamber due to poor control of engine heat, cross-firing (electrical induction between spark plug wires), or the plug heat range is too high for the engine or its operating conditions.

Spark Plug with Gap Bridging


Combustion deposits that get thrown loose may end up lodged between the electrodes. Other deposits then accumulate on top of each other until they bridge the electrode gap. The plug is unable to fire, resulting in a dead cylinder.

Splash Fouled Spark Plug

Splash Fouled

Spotted deposits appear on the firing tip of the insulator and usually happens after a long overdue engine tune-up is performed. By-products of combustion may loosen suddenly when temperatures are back to normal and during hard acceleration these materials shed from the piston crown or valve heads, and are thrown against the hot insulator surface.

Spark Plug with Detonation


Insulators appear cracked or chipped. The explosion in this case applies extreme pressure on internal engine components. Common causes include ignition time advanced too far, lean air/fuel mixture, and insufficient octane rating. Improper gap setting techniques can also result in a fractured insulator tip.

Ash Fouled Spark Plug

Ash Fouled

A build-up of combustion deposits caused mainly from the burning of oil and/or fuel additives during normal combustion. They are typically non-conductive so when heavier deposits are allowed to accumulate over time, they can “mask” the spark, resulting in a plug misfire and hesitation during acceleration.

Carbon Fouled Spark Plug

Carbon Fouled

Characterized by soft black, sooty deposits and most often caused by an over-rich air/fuel mixture. Check for a sticking choke, clogged air cleaner, or carburetor problem. This may also happen because of a weak ignition voltage, inoperative preheating system (carb intake air) or extremely low cylinder compression. Causes misfiring, hard starting and hesitation.

Worn Spark Plug


A worn spark plug not only wastes fuel but also strains the whole ignition system because the expanded gap (due to erosion) requires higher voltages. Causes hard starting in damp or cold weather and poor fuel economy.

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