MerCruiser 305 CID (5.0L) Service Manual page 110

TROUBLESHOOTING
Engine Overheats (Cooling System)
IMPORTANT: The first step is to verify if the engine is actually overheating or the
temperature gauge or sender is faulty.
IMPORTANT: Best way to test gauge or sender is to replace them.
Loose or faulty drive belt
Seawater shutoff valve partially or fully
closed (if equipped)
Clogged or improperly installed sea
strainer
Loose hose connections between seawa-
ter pickup and seawater pump inlet (mod-
els with belt driven seawater pump only)
Seawater inlet hose kinked or collapsed
Seawater pickup clogged
Obstruction on boat bottom causing water
turbulence
Defective thermostat
Exhaust elbow water outlet holes plugged
Insufficient seawater pump operation
Obstruction in cooling system such as
casting flash, sand, rust, salt, etc.
Engine circulating pump defective
IMPORTANT: In addition to previous checks, make the following checks if engine
is equipped with closed cooling.
Low coolant level
Antifreeze not mixed properly
Heat exchanger cores plugged
Water hoses reversed at the water dis-
tribution block
Index
Page 1C-28
Cause
Also refer to "Engine Overheats (Mechanical)"
SERVICE MANUAL NUMBER 24
Special Information
Pump will suck air. Pump may fail to prime
or will force air bubbles into cooling sys-
tem.
Obstruction will be in front of seawater
pickup, causing air bubbles to be forced
into cooling system
Worn pump impeller
Refer to water flow diagram for engine
type being serviced
Antifreeze should be mixed 50/50 or maxi-
mum 60/40 (60% antifreeze, 40% water)
Refer to water flow diagram in SECTION 6
90-861327--1 OCTOBER 1999