Mercury 305 CID, Service Manual, Page: 87 / 967

TROUBLESHOOTING - 1C-27 90-823225--1 1096
Engine Overheats (Cooling System)
Cause Special Information
IMPORTANT: The first step is to verify if the en-
gine is actually overheating or the temperature
gauge or sender is faulty.
Loose or broken drive belt
IMPORTANT: Best way to test gauge or sender is
to replace them.
Seawater shutoff valve partially or fully closed
(if equipped)
Clogged or improperly installed sea strainer
Loose hose connections between seawater
pickup and seawater pump inlet (models with
belt driven seawater pump only)
Pump will suck air. Pump may fail to prime or will
force air bubbles into cooling system.
Seawater inlet hose kinked or collapsed
Seawater pickup clogged
Obstruction on boat bottom causing water
turbulence
Obstruction will be in front of seawater pickup,
causing air bubbles to be forced into cooling
system
Defective thermostat
Exhaust elbow water outlet holes plugged
Insufficient seawater pump operation Worn pump impeller
Obstruction in cooling system such as casting
flash, sand, rust, salt, etc. Refer to water flow diagram for engine type
being serviced
Engine circulating pump defective
Also refer to “Engine Overheats (Mechanical)”
IMPORTANT: In addition to previous checks,
make the following checks if engine is equipped
with closed cooling.
Low coolant level
Antifreeze not mixed properly
Antifreeze should be mixed 50/50 or maximum
60/40 (60% antifreeze, 40% water)
Heat exchanger cores plugged
Water hoses reversed at the water distribution
block
Refer to water flow diagram in Section 6