Mercury 305 CID, Service Manual, Page: 85 / 967

TROUBLESHOOTING - 1C-25 90-823225--1 1096
Water In Engine
Important Information
IMPORTANT: First determine location of water in
engine. This information can be of great help
when trying to determine where the water came
from and how it got into the engine. The three
most common problems are “water on top of pis-
tons, water in crankcase oil, water in crankcase
oil and on top of pistons.”
The first step, after locating water, is to remove all the
water from the engine by removing all spark plugs
and pumping cylinders out by cranking engine over.
Next change oil and filter. Now, start engine and see
if problem can be duplicated. If problem can be dupli-
cated, there more than likely is a mechanical prob-
lem. If the problem cannot be duplicated, the problem
is either an operator error or a problem that exists
only under certain environmental conditions.
If water is contained to cylinder(s) only, it is usually
entering through the intake system, exhaust system,
or head gasket.
If the water is contained to crankcase only, it is usual-
ly caused by a cracked or porous block, a flooded
bilge, or condensation.
If the water is located in both the cylinder(s) and the
crankcase, it is usually caused by water in the cylin-
ders getting past the rings and valves, or complete
submersion.
Checking for rust in the intake manifold or exhaust
manifolds is a good idea. Rust in these areas will give
clues if the water entered these areas.
Water on Top of Pistons
Cause Special Information
Operator shut engine off at high RPM
Engine “diesels” or tries to run backwards
Engine out of tune, poor fuel, high idle RPM,
timing set too high
Rain water running into flame arrestor Hatch cover
Spark plug misfiring
Improper combustion causes moisture in the air
to accumulate in the cylinder
Backwash through the exhaust system
Improper engine or exhaust hose installation
Cracked exhaust manifold
Improper manifold to elbow gasket installation
Loose cylinder head bolts
Blown cylinder head gasket Check for warped cylinder head or cylinder block
Cracked valve seat
Porous or cracked casting
Check cylinder heads, cylinder block, and intake
manifold