The insulator foot section and electrode section are covered with dried, soft black carbon.
Results
Poor starting, misfiring, acceleration defect.
Cause
Repeated short-distance driving (driving with the engine cool), incorrect choking (overly rich air-fuel mixture), injection timing delay, plug heat range too high.
Oil fouling
Appearance
The insulator section and electrode section is black and lustrous with wet oily deposits.
Results
Poor starting and misfiring.
Cause
Oil leaking due to piston ring, cylinder, or valve guide wear (occurs easily to new engines and engines that have just been overhauled), high oil content in air-fuel mixture.(2-stroke engines)
Fuel fouling
Appearance
The spark plug is wet with gasoline immediately after it is removed, but it soon dries off.
Results
Poor starting and misfiring.
Cause
The air-fuel ratio is too rich and is not igniting. (Among the ways this can happen is if the driver presses the accelerator over and over while starting the vehicle.)
Handling
Remove all the spark plugs, crank the starter motor to bring fresh air into the cylinder and make the air-fuel ratio leaner.
Extreme electrode wear
Appearance
The center and ground electrodes are rounded and the gap has become too wide.
Results
Poor starting and acceleration.
Cause
Inadequate maintenance. (spark plug has exceeded its service life.)
Spark plug gap too large
Appearance
The gap is wider than appropriate.
Results
Poor starting and acceleration.
Cause
The gap is inappropriate; the wrong spark plug was selected.
Insulator breakage
Appearance
Insulator cracked
Results
Shorts due to insulation defect, causes poor idling and misfiring during acceleration.
Cause
Spark plug removed/installed incorrectly. (spark plug turned too far with spark plug wrench, excess tightening torque, or other inappropriate work.)
Diagnosis 2
＜Case 2＞
Misfiring only when running at high speed or when accelerating suddenly
Lead fouling
Appearance
The insulator leg section has yellow or yellowish-brown burnt on deposits or is covered with a glossy surface.
Results
Misfiring during rapid acceleration or under high load, but no problem in normal running.
Cause
Use of gasoline with much lead.
Diagnosis 3
＜Case 3＞
Loss of power when running at high speed or under high load
Piston breakdown
Overheating
Appearance
The insulator leg section is scorched extremely white with small black deposits. Rapid electrode wear.
Results
Loss of power when running at high speed or under high load.
Cause
Spark plug incorrectly tightened, engine cooling problem, ignition timing too early, spark plug heat range too low, severely abnormal combustion.
Pre-ignition
Appearance
The center electrode or ground electrode is melted or scorched. There are spots on the insulator leg section and deposits of aluminum or other metal powder.
Results
Power loss due to engine damage.
Cause
Often this is due to overheating; pre-ignition is a phenomenon in which combustion occurs before ignition. The plug heat range is too low, the injection timing is too advanced, etc.
Insulator breakage
Appearance
The insulator leg section is cracked or broken.
Results
Misfiring
Cause
Severely abnormal combustion, lack of attention to gap adjustment.
Housing installation screw section melting
Appearance
Housing installation screw section melting.
Results
Power loss due to engine damage.
Cause
Incorrect spark plug tightening.
Physical damage to ignition tip section
Appearance
The electrode is bent and the insulator leg section is broken. Indentations are sometimes seen on the electrode.
Results
Misfiring.
Cause
The spark plug thread reach is too long for the engine head or there is some kind of foreign matter (a small bolt, nut, or the like) in the combustion chamber.
Ground electrode damage & breakage
Appearance
Ground electrode is broken in the middle or at the base; much of the cross section shows fatigue fracturing.
Results
Ignition failure, damage to the engine or auxiliaries from the grounding terminal fragment.
Causes
Increased stress on the ground electrode due to high-output, high-torque engine or tuning, modification, etc.; excessive vibration due to poor maintenance, severe operating environment, etc.; insufficient fastening ⇒ excessive vibration due to loose plug
Countermeasures
(1) Spark plug selection ・ As a countermeasure to increased stress on the ground electrode, changing to a spark plug with a small ground electrode is an effective way to handle the problem from the plug side. Examples: 2-ground electrode plug, small ground electrode plug (taper cut), racing plug, surface discharge plug, etc. However, fundamentally, it is necessary to select a spark plug that matches the vehicle’s level of tuning. (2) Engine and vehicle body maintenance ・Reduction of excessive vibration through engine and vehicle body maintenance ・Tightening plugs with the recommended torque
Diagnosis 4
＜Case 4＞
Trouble other than with the engine
Corona soiling
Appearance
Brown deposits on the insulator directly above the housing
Results
No impact on the spark plug performance
Cause
This occurs due to electrical stress in the air near the insulator. (This is not a spark plug gas leak, for which it is sometimes mistaken.)
