DeVille DTS V8-4.6L VIN 9 (2002)
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Weak coils
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Worn ignition wires
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Incorrect spark plug gap
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Excessive idling or slow speeds under light loads can keep spark plug temperatures so low that normal combustion deposits may not burn off.
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Deposit fouling-Oil, coolant, or additives that include substances such as silicone, very white coating, reduces the spark intensity. Most powdery
deposits will not effect spark intensity unless they form into a glazing over the electrode.