Page 28 - REPOWER REFERENCE GUIDE (2020)

Engine Failure Diagnosis
Diagnosing Engine Failures

When a boat's engine fails, it is critical that the root cause of the failure be determined. Neglecting to isolate the root cause and
correct any discovered issues increases the likelihood of a repeat failure. Determining and correcting the cause of the failure
will save you and the customer time and money.
It is impractical to attempt to present a comprehensive manual for determining the what, when, where, why, and how of every
possible engine failure. This manual attempts only to highlight some of the more common reasons for failure, presenting key
areas of investigation. This means that you will need to rely on your own experience as a qualified marine service technician to
diagnose all of the particulars of your individual case. Use this manual only as a guide.
Remember that customer input is key to diagnosing the cause of any problems with the boat's engine, including engine failure.
Be certain to obtain and evaluate the following customer‑supplied information:
• Experience at the time of the failure
• How the customer normally operates the boat
• Engine RPM at wide‑open throttle (WOT)
• Boat speed
• Maintenance records
Other key areas to investigate include:
• Exhaust components (engine and boat)
• Ignition system
• Fuel system (engine and boat)
• Electrical system (engine and boat)
• Cooling system (engine and boat)
• Engine, including:
• External condition
• Cylinder compression and leak down
• Internal inspection
• Boat condition (for example: bottom growth)
• Condition of sterndrive, inboard running gear, and propellers
NOTE: The following subsections—Used Spark Plug Analysis, Bearing Failures, and Piston Failures—provide in‑depth
analysis for determining some common engine malfunctions.
Determine Cause of Failure

Why It Is Important

IMPORTANT: A proper repair begins with a root cause determination.
• Mercury remanufacturing warranty returns analyzed.
• It is always important to know why a failure occurred for a proper repair. Sometimes this is easy: like in the case of freeze
damage. The water in the block was not drained, it froze, expanded, and will crack the cast iron. As long as the water gets
drained, the engine will not crack.
• It is a common mistake for a symptom to be mistaken for the cause of a failure. Some people may think an engine failed
because a piston melted. That can lead to a course of action to change the engine, assume it is fixed because the melted
piston was changed, and then wonder why it happens again. A failed piston is almost never the cause, but it is definitely a
symptom that something went wrong. Something causes a piston to melt. Don't fix symptoms: seek to fix the root cause of
the issue or it will reoccur.
• Example:
A 496 MAG HO engine fails after years of use.
The first remanufactured longblock is ordered to replace the engine.
The first remanufactured longblock fails in the first hour of use.
A second remanufactured longblock is ordered to replace the first remanufactured longblock replacement.
The second remanufactured longblock fails in the first hour of use.
The dealer now begins to determine why the failure occurred.
A faulty fuel injector is found to cause the lean operation, detonation, and the piston failure.
A third remanufactured longblock is installed. The injectors are cleaned. The boat runs well after $20,000 is spent on
engines.

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