Technical information informatio n Spark Plug Analysis
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. 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 con ductive 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 4! " #$!%C, 4!%C is the spark plug self cleaning temperature at which point carbon deposits will burn off. &owever, if too cold a spark plug is used and this temperature is not achieved carbon fouling will occur. This is the most common reason for carbon fouling.
'ther causes for carbon fouling include
Causes
Air/fuel mixture A/F! too rich
Corrective actions
*
+ncorrect adustment of carburetor +t is necessary to service the carburetor, the a choke system or the fuel inection system. -aulty auto choke system
-aulty fuel inection system
Faulty electrical system
* +t is necessary to service the electric system
oor connection of high tension cables
"na#e$uate running con#itions
* +t is necessary to run at higher speeds (about km/h) from time to time.
rolonged idling
Continuous low speed driving
Too col# a spark plug
* 0se a hotter spark plug (12ample 3 561788 77* 3 51788)
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. 9hen the generated voltage becomes lower than the re:uired voltage of a spark plug (the voltage needed to cause sparks at the spark gap), sparking is suppressed and mis7firing occurs.
% : Terminal &ut 'ear
12cessive vibration of the engine may lead to abnormal wear of the terminal nut. As a result, the cover may come off the plug. -or vehicles whose engines vibrate more than others, such as watercraft and snowmobiles, solid post terminal plugs with e2cellent vibration resistance and wear resistance are recommended.
( : Flash)*ver ) Spark leakage from terminal to metal shell Flash)*ver 9hen the spark gap has widened due to wear of the electrodes, a higher voltage is re:uired. The flash7over occurs when the re:uired 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 flash7over.
As a higher voltage is re:uired for a turbo charged engine, flash7over is more likely to occu r. +t is important to recognise that a plug cable is a consumable part which needs to be replaced
periodically. 9hen 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. +t is not a stain caused by gas leakage but by corona discharge (Carona ;tain).
,echanism of corona #ischarge
The high voltage applied to the spark gap is also applied to the area between the centre electrode and the metal shell, causing an insulation breakdown of the air at the gap (a) between the insulator and the metal shell. The phenomena is called a corona discharge. The generated carona discharge develops toward the terminal nut. This last process is the pale blue carona discharge that is observed at night.
- : ,etal shell ) .ust breakage at caulke# portion
9hen 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 inade:uate draining through the plug hole,
5ust the metal shell may rust.
The Caulked ortion
The rusting of the metal shell causes no deterioration of the function of the spark plug.
3roken Ceramics
>elted 1lectrodes
?eposits
0 : Firing n# ) 2roken ceramics melte# electro#es #eposits 9hen the firing end of the plug has overheated, the ceramics may break or the electrodes may melt. 0nder usual engine condition, the plug does not overheat.
9hen deposits have accumulated on the firing end of the plug, deposits may overheat, causing abnormal combustion. 1specially in a two 7 c ycle engine, oil gets burned and remains in the combustion chamber as deposits, accumulating on the plug as well. +t is necessary to remove these deposits periodically. +n engines that consume larger amounts of oil, oil may enter the combustion chamber. +t is necessary to check the amount of deposits during inspection of the plug. 3urning of oil can also be detected by visible white e2haust gas emitted from the tail pipe.
3 : 4ry an# 5et fouling 9et 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. lease see 8. Carbon -ouling
6 : "nsulator ) 2reakage at the corrugation an# caulke# portion
Caulked portion Corrugation 9hen removing or installing the plug, the plug wrench may slip or be tilted and may hit the corrugation of the plug against the inside, b reaking the insulator. ?epending 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. ;lipped wrench leaves a mark on the he2agonal portion of the metal shell. A plug wrench with a definite he2agonal shape should be used. A rounded or loose wrench should be either avoided or used with special care.
7 : 8n#er or over tightening
Too tight Too loose 9hen 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. +f the metal shell continues to overheat the plug temperature may rise and abnormal combustion may result. 9hen the plug has been tightened too much the thread neck portion of the metal shell will be lengthened. 9hen the threaded neck portion is lengthened, the insulator and metal will not seal tight enough, causing the combustion gas to leak. 9hen the gas continues to leak, the plug will be overheated and abnormal combustion may result. The plug should be tightened to the following recommended tor:ues and turning angles.
9hen the plug is tightened too much the metal shell thread neck may be broken at around the first to second thread. The plugs with smaller diameter portions, the ? (8@ mm) or C (8! mm) types thread neck portion is not very strong. 'bserve the recommended tor:ue and turning angle and tighten with special care.
19 : Plug socket troubles ?amage to spark plugs during installation can easily occ ur and will oftern result in an intermittent misfire or no spark at all. Care must be taken during installation that the spark plug socket is not slanted and does not slip causing insulator breakage (;ee -igure 8). +nternal damage can still occur without the insulator actually breaking. Any hori=ontal force can either bend the terminal post internally or result in internal cracking of the insulator. To avoid plug damage the following :ualities are re:uired in a wrench (;ee -igure @)
8. That it fits the spark plug Bhe2B properly. @. The inner space must be large enough to avoid contact with the insulator. . The spark plug socket must completely cover the Bhe2B portion of the metal shell. 4. A he2agonal socket is preferable as it is less likely to slip than a twelve point star type.
11 : xamples of problems cause# by excessively over long perio#s of use
?eposits accumulated on the firing end may induce abnormal combustion (pre7ignition), causing problems that include melting of the electrodes.
oo#
'orn lectro#es
+f the edges of electrodes are worn and rounded, sparks will not easily occur, which lea engine starting problems and mis7firing during running ma2imum power cannot b e obtained from the engine.
Foule#
ower leaks through the carbon accumulated on the firing end, causing mis7firing as we hindering engine starting.
4eposits
The deposits accumulated on the spark plug overheat and cause abnormal combustion ( ignition) that may lead to melting of the electrodes of the spark plug.
'hy is the firing en# appearance of a spark plug important;
3ecause the -iring end appearance reflects the suitability of the spark plug as well as the condition of the engine. There are three basic criteria good, fouled, and overheated. The firing end appearance also depends on the spark plug tip temperature.
The boarder7line between the fouling and optimum operating regions (4! degrees Celsius) is called the spark plug self cleaning temperature. +t is at this temperature that the deposits accumulated are burnt off. 'hat is spark plug fouling;
+t is a phenomenon by which carbon attached to the firing end causes electrical leakage that leads to mis7firing. As the high voltage generated by the ignition coil leaks away through the carbon, mis7firing may occur and cause running and starting difficulties.
'hat is spark plug overheating;
rolonged overheating may induce abnormal combustion (pre7ignition), resulting in melting of the spark plug electrodes. 'verheated spark plugs have a white insulator surface at the firing end speckled dep osits. 1lectrode melting represents e2cessive overheating. 9hen the spark plug temperature e2ceeds #$!%C, the firing end serves as a heat source before sparking and induces abnormal combustion (pre7ignition), possibly damaging the piston.
Causes of overheating an# corrective actions
Causes
Corrective actions
+gnition timing too far advanced
*
Adustment of ignition timing is re:uired.
Air/fuel mi2ture(A/-) too lean
*
Adustment of air fuel ratio (A/f) is re:uired.
+nsufficient cooling water and lubricants
*
Top up cooling water and lubricants
Applied turbo boost pressure too high in the case of a * turbo engine
Adustment of turbo boost pressure control is re:ui
+nsufficient tightening of spark plug
*
Tighten to specified tor:ue
*
0se colder spark plug (12ample 3 51788 77* 3 561788)
0se of too hot a spark plug
'hat is lea# fouling;
Dead deposits accumulated on the spark plug firing end help electricity to leak away and result in mis7firing. 9hen the lead contained in gasoline as an octane enhancer, adheres as the firing end of the spark plug, the high voltage generated by the ignition leaks through the lead deposits and causes mis7 firing during acceleration. This problem often takes @,!!! E ,!!! km (8,@!! E 8,#!! miles) to become apparent.
"nsulation resistance of a spark plug vs< vehicle spee#< Dead fouling does not appear during starting or low speed running. 'n the other hand, mis7firing occurs when accelerating from the middle speed range since the insulation resistance of the spark plug drops :uickly.
2etter selection of spark plugs<
0se hotter type (12ample 361; 7 31;) 0se proected insulator nose type (12ample 361; 7 361;)
