Table of Contents Table of Contents
Previous Page  566 / 612 Next Page
Show Menu
Previous Page 566 / 612 Next Page
Page Background

By examining the insulator iring nose color, an experienced engine tuner can determine a great deal about the engine’s overall operat


ing condition.

In general, a light tan/gray color tells you that the spark plug is operating at optimum temperature and that the engine is in good con


dition. Dark coloring, such as heavy black wet or dry deposits, can indicate an overly rich condition, to cold a heat range spark plug, a

possible vacuum leak, low compression, overly retarded timing or to large a plug gap.

If the deposits are wet, it can be an indication of a breached head gasket, poor oil control from ring or valve train problems or an ex


tremely rich condition—depending on the nature of the liquid present at the iring tip.

Signs of fouling or excessive heat must be traced quickly to prevent further deterioration of performance and possible engine damage.

Normal Condition

An engine’s condition can be judged by the

appearance of the spark plug’s iring end.

If the iring end of a spark plug is brown or

light gray, the condition can be judged to

be good and the spark plug is functioning


Dry and Wet Fouling

Although there are many different cases,

if the insulation resistance between the

center electrode and the shell is over 10

ohms, the engine can be started normal


ly. If the insulation resistance drops to 0

ohms, the iring end is fouled by either wet

or dry carbon.


When a spark plug overheats, depos


its that have accumulated on the insu


lator tip melt and give the insulator tip

a glazed or glossy appearance.


The accumulation of deposits on the

iring end is inluenced by oil leakage,

fuel quality and the engine’s operating


Lead Fouling

Lead fouling usually appears as yellowish

brown deposits on the insulator nose. This

cannot be detected by a resistance tester at

room temperature. Lead compounds combine

at different temperatures. Those formed at

370-470°C (700-790°F) have the greatest

inluence on lead resistance.


Breakage is usually caused by ther


mal expansion and thermal shock

due to sudden heating or cooling.

Normal Life

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.

Abnormal Erosion

Abnormal electrode erosion is

caused by the effects of corro


sion, oxidation and reaction with

lead—all resulting in abnormal gap



Melting is caused by overheating. Mostly,

the electrode surface is rather lustrous and

uneven. The melting point of nickel alloy is

1,200~1,300°C (2,200~2,400°F).

Erosion, Corrosion and Oxidation

The material of the electrodes has oxi


dized, and when the oxidation is heavy,

it will be green on the surface. The

surface of the electrodes is also fretted

and rough.

How to read a spark plug?


Lead Erosion

Lead erosion is caused by lead

compounds in the gasoline which react

chemically with the material of the elec-

trodes (nickel alloy) as high temperatures;

crystal of nickel alloy fall off because of the

lead compounds permeating and separat-

ing the grain boundary of the nickel

alloy. Typical lead erosion causes the surface of the

ground electrode to become thinner, and the tip of the

electrode looks as if it has been chipped.