1999 Mercruiser HI-Performance GM 377 EFI Engine Service Manual, Page 48Get this manual

1999 Mercruiser HI-Performance GM 377 EFI Engine Service Manual, Page 48

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TROUBLESHOOTING

Overheating
Insulator is dull white or gray and appears blisteredElectrodes are eroded and there is an absence of depositsCheck that correct plug heat range is being