A overheating outboard can happen for a variety of reasons. If you have ever noticed a lack of water coming
from the cooling water pilot hole (also called the
“overboard discharge indicator” or ODI or “pisser” or telltale) on your
outboard, here are a few tips that can help
you quickly analyze the situation.
TIP 1: Ensure that the lower unit water inlets are clean
and installed correctly. When those are confirmed,
lower the engine into the water until the water pump
is well-submerged and start the engine. Place your
hand near the exhaust outlet in the back of the
engine and feel the temperature of the exhaust.
TIP 2: Check and see if the pilot hole is smoking. Is it
blowing off steam from residual water in the block or
is it exhaust smoke? If it is exhaust smoke, this can
indicate a compromised head gasket or poor sealing
at the powerhead base or at the side plate gaskets.
Ask yourself these questions:
• When was the last water pump replaced?
• Why was it replaced?
• Are there broken blades lodged in the
powerhead?
• Was the impeller replaced?
• Is the housing melted internally near the outlet?
NOTE: This can cause water restriction.
Other points to check include the following:
• The water copper tube has corroded off on the
end at the water pump housing.
• The water tube has a few holes.
• The absence of a seal in the water pump housing
for the water tube.
Any of the above situations can drain water and
induce exhaust into the cooling passages. If you find
that the water pump, impeller, and water tube are in
good condition, then there is a good chance that the
pilot hole has a blockage.
When the pilot hole has a blockage, the typical sign is
the exhaust temperature feeling cool and a little damp,
which can be an indication that water is entering the
block. If water is getting to the block and the exhaust
is cool, check to see if a spider has crawled inside and
made a small web. This is very common and the web
can collect dirt and debris, clogging the component
solid. Remove the hose to the pilot hole to see if water is
emanating from it.
If there isn’t any water coming from the hose
connected from the pilot hole, then check the other
areas where water is supplied to the pilot hole.
Depending on the outboard, water can be supplied to
the pilot hole from different areas, such as the cooler
cover or thermostat. The cooler cover can become
clogged with debris as well as thermostats. Because
thermostats are designed to allow some water and air
to escape from the block, they do not seal completely
which allows the possibility for that point to become
clogged as well. However, if there is water coming
from the hose, simply grab a zip tie or weed-eater
string to clean out the pilot hole.
Many times, compressed air is used
to try to clear a clogged system on the block.
However, keep in mind that you may blow off a
waterline elsewhere on the engine which can lead
to a catastrophic costly result from the engine
running, while getting soaked with water.
Always perform a visual inspection with the
engine running at all speeds after using high
pressure air to clear a suspected clog.
The other item to consider is the poppit valve. If this
valve is stuck open, water may not totally fill the block
as it would be when drained out. The poppit valves
(should the engine have one) serve as a relief of
coolant pressure, thus restricting water volume to the
block so that the thermostat(s) will work correctly. If
the poppit is stuck closed, then this will overpressure
the block, causing an overcooling of the engine at
higher rpm. This will cause a very strong stream of
water to come out of the pilot hole.
Finally, remember, when flushing the motor using
the flush muff method, a lack of water pressure or
volume from the hose can be a contributing factor. As
the inlets have very small openings, the hose needs
to supply enough water pressure and volume so that
the water pump does not cavitate. If it does, it will not
pick up enough water to supply the engine. The best
procedure is to allow the engine to run in its natural
environment—the body of water where the engine
resides or within a bucket.
The pilot hole is a very important indicator when
troubleshooting engines for suspected overheating
or over-cooling. Once you fully understand the
operational and visual aspects of the pilot hole,
troubleshooting is quite easy.