TROUBLESHOOTING SERVICE MANUAL NUMBER 23
Page 1C-24
90-861326--1 MARCH 1999
Engine Overheats
Mechanical
Cause Special Information
Engine rpm below specifications at wide-
open-throttle (engine laboring)
Damaged or wrong propeller; growth on
boat bottom; false bottom full of water
Wrong ignition timing Timing too far advanced or retarded
Sticking distributor advance weights
Spark plug wires crossed (wrong firing
order)
Lean fuel mixture
Wrong heat range spark plugs
Exhaust restriction
Valve timing off
Jumped timing chain, or improperly
installed
Blown head gasket(s)
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.
Insufficient lubrication to moving parts of
engine
Defective oil pump, plugged oil passage,
low oil level
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