Overheating; High Speed Glazing; Scavenger Deposits - Mercury MerCruiser Service Manual

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Engine Troubleshooting

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. Replace the spark plugs.

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 colder heat range plug and service plugs more frequently.

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. 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.

90-8M0099748 eng DECEMBER 2015

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