Cold Fouling; Overheating; High Speed Glazing - Mercury MerCruiser MCM 454 Mag MPI Service Manual

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SERVICE MANUAL NUMBER 23

Cold Fouling

Dry, black deposits indicate rich fuel mixture or weak ignition. Clogged flame arrestor or

weak ignition components all are probable causes. If, however, only one or two plugs in set

are fouled, check for sticking valves or bad ignition leads. After correcting cause, service

(clean, file and re-gap) plugs and reinstall.

Insulator is dull white or gray and appears blistered. Electrodes are eroded and there is an

absence of deposits. Check that correct plug heat range is being used. Also check for

over-advanced ignition timing, cooling system malfunction, lean fuel/air mixtures, leaking

intake manifold or sticking valves. Replace spark plugs.

Insulator has yellowish, varnish-like color, indicating that 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 spark plugs.

If condition recurs, use colder heat range plug and service plugs more frequently.

90-861326--1 MARCH 1999

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