Preignition Damage
Preignition damage is caused by excessive high temperatures. The center electrode melts first, followed by the ground
electrode. Normally, the insulators are white, but may be dirty if the plug has been misfiring. Check for the correct plug heat
range, incorrect rotor location, lean fuel mixture, incorrect fuel used, malfunctioning cooling system, leaking intake manifold, or
lack of lubrication. Replace the spark plugs.
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Reversed Coil Polarity
Concave erosion of the ground electrode is an indication of reversed polarity. The center electrode will show only normal wear.
The engine will misfire and idle rough. To correct, reverse the primary coil leads. Replace the spark plugs.
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Splashed Deposits
Splashed deposits, which sometimes occur after long delayed tune‑up, accumulate after a long period of misfiring. When
normal combustion temperatures are restored, upon the installation of the new plugs, the deposits loosen from the top of the
piston and the cylinder head and are thrown against the hot insulator. Service (clean, file, and regap) plugs and reinstall.
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Engine Troubleshooting
90-8M0099748 eng DECEMBER 2015 © 2016 Mercury Marine Page 3C-7
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