2001 Mercury Mariner 50-60HP Factory Service Manual, Page 20Get this manual
MAINTENANCE
Flushing the Cooling System
Flush the internal water passages of the outboard with fresh water after each use in salt, polluted or muddy waterThis will help prevent buildup of deposits from clogging the internal water passagesIMPORTANT: The engine must be run during flushing in order to open the thermostat and circulate water through the water passages
NOTE: You can have the outboard tilted or in the vertical operating position during flushing
WARNING