Mechanical Damage
Mechanical damage to the spark plug firing end is caused by a foreign object in the combustion chamber. Because of valve
overlap, small objects can travel from one cylinder to another. Check all cylinders, the intake manifold, and the exhaust material
to prevent further damage. Replace the spark plugs.
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IMPORTANT: When working on the engine, the spark plug holes, the intake, and the throttle body should be kept covered to
prevent foreign objects from entering the combustion chamber.
Piston Inspection
1. Replace pistons that are damaged or show signs of excessive wear.
2. Inspect the piston for damage caused by preignition.
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Typical preignition damage
a - Preignition begins
b - Controlled ignition spark
c - Remaining fuel ignition
d - Flame front collision
NOTE: Engine failures that result from these conditions are beyond the control of Mercury MerCruiser. No warranty will
apply to failures that occur under these conditions.
3. If indicated, inspect the engine for causes of preignition damage to pistons:
• Combustion chamber hot spots caused by damaged components, foreign material, or combustion chamber deposits.
Engine Troubleshooting
Page 3C-8 © 2016 Mercury Marine 90-8M0099748 eng DECEMBER 2015
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