Page 39 - REPOWER REFERENCE GUIDE (2020)

Engine Failure Diagnosis

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. Perform a compression check on all cylinders. 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 Failures

Preignition
Preignition is abnormal fuel ignition caused by combustion chamber hot spots. Control of the start of ignition is lost, as
combustion pressure rises too early, causing power loss and rough running. The upward motion on the piston is opposed by
the pressure rise. This can result in extensive damage to the internal parts from the increase in combustion chamber
temperature. Preignition events occur suddenly and without audible warning. Engine noise may result after the event from a
severely damaged engine.
Preignition Damage

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90-8M0149179 eng NOVEMBER 2018 © 2019 Mercury Marine Page 3A-13