ignition system

fault diagnosis sheet - spark plugs

Appearance of a used spark plug tells a lot about the operating conditions of the engine and the plug. Thus the analysis of a plug plays an important role in the maintenance of a car.



1 : Carbon Fouling
Carbon fouling is the most common spark plug related failure, but is not a spark plug fault. Carbon deposits are conductive, and as they accumulate along the insulator nose they reduce the insulation resistance of the spark plug. As electricity always takes the path of least resistance a misfire may occur if a significant amount of carbon deposits accumulate. A spark will not form as electricity can track along the conductive carbon deposits to the metal shell (as shown in red below) rather than forming a spark across the electrode gap which has a very high resistance.



As mentioned the optimal operating temperature range for a spark plug is 450 – 870°C, 450°C is the spark plug self cleaning temperature at which point carbon deposits will burn off. However, if too cold a spark plug is used and this temperature is not achieved carbon fouling will occur. This is the most common reason for carbon fouling.



Other causes for carbon fouling include:

Causes

Corrective actions

Air/fuel mixture (A/F) too rich
==>
It is necessary to service the carburetor, the auto choke system or the fuel injection system.

Incorrect adjustment of carburetor


Faulty auto choke system


Faulty fuel injection system


Faulty electrical system
==>
It is necessary to service the electric system.

Poor connection of the high tension cables


Inadequate running conditions
==>
It is necessary to run at higher speeds (about 80 km/h) from time to time.

Prolonged idling


Continuous low speed driving


Too cold a spark plug
==>
Use a hotter spark plug
(Example: BK R6E-11 --> BK R5E-11)

Air cleaner contaminated



As carbon builds up, the insulation resistance of the spark plug drops and the voltage generated by the ignition coil is reduced. When the generated voltage becomes lower than the required voltage of a spark plug (the voltage needed to cause sparks at the spark gap), sparking is suppressed and mis-firing occurs.

2 : Terminal Nut Wear
Excessive vibration of the engine may led to abnormal wear of the terminal nut. As a result, the cover may come off the plug. For vehicles whose engines vibrate more than others, such as watercraft and snowmobiles, solid post terminal plugs with excellent vibration resistance and wear resistance are recommended.

Solid post terminal plugs are currently available with five part numbers: B7ES, B8ES, and BR9ES. A special package has been made for solid post terminal plugs.

3 : Flash-Over - Spark leakage from terminal to metal shell

Flash-Over When the spark gap has widened due to wear of the electrodes, a higher voltage is required. The flash-over occurs when the required voltage between the plug electrodes is higher than the voltage flying between the terminal and metal shell.
The plug cable material hardens as time elapses, which in turn reduces the tightness of the cover and insulator, lowering the preventive power for flash-over.

As a higher voltage is required for a turbo charged engine, flash-over is more likely to occur. It is important to recognise that a plug cable is a consumable part which needs to be replaced periodically. When there is no spark after washing the car or the engine room, check whether water has entered the plug cover or not.




Corona Stain A removed spark plug sometimes has discolouration around the insulator surface which looks like gases have leaked between the insulator and the shell. This discolouration is generally called " Corona stain" and is caused by the fact that oil particles in the air around the insulator surface are attracted by the corona discharge, are charged, and then adhere to the insulators surface. The corona stain causes no deterioration of the function of the spark plug.


4 : Metal shell - Rust, breakage at caulked portion

Rust
The Caulked Portion
When water has entered the plug hole due to water resistance of the plug cover or, in the case of a motorcycle, when water has accumulated due to the inadequate draining through the plug hole, the metal shell may rust.

The rusting of the metal shell causes no deterioration of the function of the spark plug. Note, however, that water inside the cover may prevent sparks from being generated.

If the plug is forced to remove when the returning torque is abnormally high due to some causes such as plug thread seizure, the plug may break at the caulked portion of the metal shell.

When returning torque is high, the engine should be first warmed up. Then, by spraying penetrating liquid around the plug thread and leaving it for a while, the plug can sometimes be removed more easily.

5 : Firing End - Broken ceramics, melted electrodes, deposits

Broken Ceramics Melted Electrodes Deposits
When the firing end of the plug has overheated, the ceramics may break or the electrodes may melt.
Under usual engine condition, the plug does not overheat. Note, however, that it gets extremely hot in the case of abnormal combustion (ex. high - speed knocking, pre - ignition).

When the A - F setting is lean due to a faulty fuel system, the combustion temperature may rise, resulting in abnormal combustion. The engines cooling system may be faulty. When the spark timing is too early, the combustion temperature may rise, resulting in abnormal combustion. When deposits (generating from combustion) are accumulated in the combustion chamber, the combustion temperature may rise, resulting in abnormal combustion.

When deposits have accumulated on the firing end of the plug, deposits may overheat, causing abnormal combustion. Especially in a two - cycle engine, oil gets burned and remains in the combustion chamber as deposits, accumulating on the plug as well. It is necessary to remove these deposits periodically.

In engines that consume larger amounts of oil, oil may enter the combustion chamber. It is necessary to check the amount of deposits during inspection of the plug. Burning of oil can also be detected by visible white exhaust gas emitted from the tail pipe.

6 : Dry and wet fouling
Wet fouling is fundamentally similar to carbon fouling. Although the root cause may vary due to a number of reasons, in essence the insulation resistance is reduced allowing a spark to track along the insulator nose and earth to the metal shell rather than forming a spark across the electrode gap as desired. Please see 1. Carbon Fouling

7 : Insulator - Breakage at the corrugation and caulked portion

Corrugation
Caulked portion
When removing or installing the plug, the plug wrench may slip or be tilted and may hit the corrugation of the plug against the inside, breaking the insulator.
Depending on the type of plug wrench, the plug may break at the caulked portion of the metal shell. This may not be noticeable from the outside appearance.

Slipped wrench leaves a mark on the hexagonal portion of the metal shell. A plug wrench with a definite hexagonal shape should be used. A rounded or loose wrench should be either avoided or used with special care.

8 : Gasket too loose, too tight

Too loose
Too tight
When the plug has not been tightened enough, the combustion gas leaks out the thread portion. This reduces the radiation of the plug, causing the metal shell to be heated, resulting in discolouration of the metal shell plating. If the metal shell continues to overheat the plug temperature may rise and abnormal combustion may result.
When the plug has been tightened too much the thread neck portion of the metal shell will be lengthened. When the threaded neck portion is lengthened, the insulator and metal will not seal tight enough, causing the combustion gas to leak. When the gas continues to leak, the plug will be overheated and abnormal combustion may result.
The plug should be tightened to the following recommended torques and turning angles.