Page 37 - REPOWER REFERENCE GUIDE (2020)

Engine Failure Diagnosis

Overheating
The insulator is dull white or gray and appears blistered. The electrodes are eroded and there is an absence of deposits.
Ensure that the correct plug heat range is being used. Also check for incorrect rotor location, cooling system malfunction, lean
fuel and air mixtures, leaking intake manifold, or sticking valves. Perform a compression and leak down check on all cylinders.
Replace the spark plugs.

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High Speed Glazing

The insulator has yellowish, varnish‑like color, indicating that the temperatures suddenly have risen, usually during hard, fast
acceleration under heavy load. Normal deposits do not get a chance to blow off. Instead, they melt and form a conductive
coating. Replace the spark plugs. If the condition recurs, use a colder heat range plug and service plugs more frequently.

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Scavenger Deposits

Powdery white or yellow deposits are built up on the shell, the insulator, and the electrodes. This is the normal appearance with
certain branded fuels and typical of MTBE fuel additive/oxygenate deposits. Accumulation on the ground electrodes and the
shell areas may be unusually heavy, but may be easily chipped off. Service (clean, file, and regap) plugs and reinstall.

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90-8M0149179 eng NOVEMBER 2018 © 2019 Mercury Marine Page 3A-11