Mercury 305 CID, Service Manual, Page: 86 / 967

1C-26 - TROUBLESHOOTING 90-823225--1 1096
Water in Crankcase Oil
Cause Special Information
Water in boat bilge
Boat has been submerged or bilge water was
high enough to run in through dipstick tube
Water seeping past piston rings or valves Refer to “Water in Engine” (“On Top of Pistons”)
Engine running cold
Defective thermostat, missing thermostat;
prolonged idling in cold water
Intake manifold leaking near a water passage
Cracked or porous casting
Check cylinder head, cylinder block, and intake
manifold
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 Refer to “Carburetor Malfunctions” in this section
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. Normally the
engine will run at normal temperature 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