Scavenger Deposits; Pre-Ignition Damage; Reversed Coil Polarity - Mercury MerCruiser MCM 454 Mag MPI Service Manual

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TROUBLESHOOTING

Scavenger Deposits

Powdery white or yellow deposits are built up on shell, insulator and electrodes. This is

normal appearance with certain branded fuels. Accumulation on ground electrodes and

shell areas may be unusually heavy, but may be easily chipped off. Plugs can be serviced

(clean, file and re-gap) and reinstalled.

Pre-ignition damage is caused by excessive high temperatures. Center electrode melts

first, followed by ground electrode. Normally, insulators are white but may be dirty if plug has

been misfiring. Check for correct plug heat range, advanced ignition timing, lean fuel

mixture, incorrect fuel used, malfunctioning cooling system, leaking intake manifold or lack

of lubrication. Replace spark plugs.

Concave erosion of ground electrode is an indication of reversed polarity. Center electrode

will show only normal wear. Engine will misfire and idle rough. To correct, reverse primary

coil leads. Replace spark plugs.

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SERVICE MANUAL NUMBER 23

90-861326--1 MARCH 1999

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