NGKsparkplug Tech-Tips page1

			NORMAL CONDITION- An engine's condition can be judged by the spark plugs firing end's appearance. If the firing end of a spark plug is brown or light gray, the condition can be judged to be good and the spark plug is functioning correctly.

			DRY & WET FOULING- Although there are many different causes, if the insulation resistance between the center electrode and the shell is over 10M, the engine can be started normally in all cases. If the insulation resistance drops to 0, the firing end is fouled by either wet or dry carbon. Causes- Too rich air/fuel mixture. Unsuitable heat range. Excessive idling or low speed driving. Ignition timing lag. Irregular spark gaps.

			OVERHEATING- When having been overheated, the insulator tip is glazed or glossy, and deposits which have accumulated on the insulator tip have melted. Sometimes these deposits have blistered on the insulator's tip. Causes- Heat range of spark plug is too low. Ignition timing is too advanced. Low octane number of gasoline is being used. Insufficient cooling. Lean air/fuel mixture. Accumulated deposit in combustion chamber. Insufficient tightening of plug.

			LEAD FOULING- Lead fouling usually appears as yellowish brown deposits on the insulator nose and this can not be detected by a resistance tester at room temperature. Lead compounds combine at different temperatures; those formed at 370~420 C ( 700~790 degrees F ) having the greatest influence on the resistance.

			DEPOSITS- The accumulation of deposits on the firing end is influenced by oil leakage, fuel quality and the engine's operating period.

			NORMAL LIFE- A worn spark plug not only wastes fuel but also strains the whole ignition system because the expanded gap requires higher voltages.


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