Engine Overheats; Mechanical - Mercury MerCruiser MCM 454 Mag MPI Service Manual

TROUBLESHOOTING

Engine Overheats

Mechanical

Engine rpm below specifications at wide-
open-throttle (engine laboring)
Wrong ignition timing
Sticking distributor advance weights
Spark plug wires crossed (wrong firing
order)
Lean fuel mixture
Wrong heat range spark plugs
Exhaust restriction
Valve timing off
Blown head gasket(s)
Insufficient lubrication to moving parts of
engine
Index Index
Page 1C-24
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Cause
SERVICE MANUAL NUMBER 23
Special Information
Damaged or wrong propeller; growth on
boat bottom; false bottom full of water
Timing too far advanced or retarded
Jumped timing chain, or improperly
installed
A blown head gasket(s) normally cannot
be detected by a compression check. Nor-
mally the engine will run at normal temper-
ature at low rpm, but will overheat at
speeds above 3000 rpm.
Engines that are seawater cooled:
Using a clear plastic hose, look for air
bubbles between seawater pump and
engine. If there are no bubbles present,
install clear plastic hose between
thermostat housing and manifold(s). If air
bubbles are present at a higher rpm, it is a
good indication there is a blown head
gasket.
Defective oil pump, plugged oil passage,
low oil level
90-861326--1 MARCH 1999