To use all available functions on this website, JavaScript must be enabled in your browser.

Troubleshooting – DENSO SPARK PLUG


Diagnosing the state of the spark plugs is an effective method for elucidating the cause of engine troubles.


The insulator leg section has light gray or tan deposits and slight electrode erosion.


The flowing trouble can be caused by engine defects.

Engine trouble

Case 1Starting defect
Misfiring when idling
Misfiring during normal running
Diagnosis 1
Case 2Misfiring only when running at high speed or when accelerating suddenlyDiagnosis 2
Case 3Loss of power when running at high speed or under high load Piston breakdownDiagnosis 3
Case 4Trouble other than with the engineDiagnosis 4

Spark plug state

Diagnosis 1

<Case 1>
  • Starting defect
  • Misfiring when idling
  • Misfiring during normal running
Carbon fouling
The insulator foot section and electrode section are covered with dried, soft black carbon.
ResultsPoor starting, misfiring, acceleration defect.
CauseRepeated 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
The insulator section and electrode section is black and lustrous with wet oily deposits.
ResultsPoor starting and misfiring.
CauseOil 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
AppearanceThe spark plug is wet with gasoline immediately after it is removed, but it soon dries off.
ResultsPoor starting and misfiring.
CauseThe 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.)
HandlingRemove 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
The center and ground electrodes are rounded and the gap has become too wide.
ResultsPoor starting and acceleration.
CauseInadequate maintenance. (spark plug has exceeded its service life.)
Spark plug gap too large
AppearanceThe gap is wider than appropriate.
ResultsPoor starting and acceleration.
CauseThe gap is inappropriate; the wrong spark plug was selected.
Insulator breakage
Insulator cracked
ResultsShorts due to insulation defect, causes poor idling and misfiring during acceleration.
CauseSpark 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
The insulator leg section has yellow or yellowish-brown burnt on deposits or is covered with a glossy surface.
ResultsMisfiring during rapid acceleration or under high load, but no problem in normal running.
CauseUse of gasoline with much lead.

Diagnosis 3

<Case 3>
  • Loss of power when running at high speed or under high load
  • Piston breakdown
The insulator leg section is scorched extremely white with small black deposits. Rapid electrode wear.
ResultsLoss of power when running at high speed or under high load.
CauseSpark plug incorrectly tightened, engine cooling problem, ignition timing too early, spark plug heat range too low, severely abnormal combustion.
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.
ResultsPower loss due to engine damage.
CauseOften 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
The insulator leg section is cracked or broken.
CauseSeverely abnormal combustion, lack of attention to gap adjustment.
Housing installation screw section melting
Housing installation screw section melting.
ResultsPower loss due to engine damage.
CauseIncorrect spark plug tightening.
Physical damage to ignition tip section
The electrode is bent and the insulator leg section is broken. Indentations are sometimes seen on the electrode.
CauseThe 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
Ground electrode is broken in the middle or at the base; much of the cross section shows fatigue fracturing.
ResultsIgnition failure, damage to the engine or auxiliaries from the grounding terminal fragment.
CausesIncreased 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
Brown deposits on the insulator directly above the housing
ResultsNo impact on the spark plug performance
CauseThis 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.)