wavdancr

[wavdancr]

I installed a new thermostat recently. The engine runs at 160 degrees in flat water. When I run the engine at sea, however, the temperature rises to 200+ degrees under load. I take it out of gear and run it at 2000 rpm and it cools to 180 degrees.
I noticed a little wear on the impeller shaft when I replaced the impeller.
Is it possible that air bubbles are preventing the water pump from pumping enough water while the boat is moving? Should I install a new water pump assuming that the pump is worn?
Thanks, Bob

[Don Moyer]

Bob,

I don't believe your overheating problem has anything directly to do with a
"little wear" on the impeller shaft.

If the problem just began after installing a new thermostat, it could of
course be a bad thermostat from the factory, or (somewhat more likely) your
water pump may be a bit on the anemic side and can't provide sufficient
water flow with an operable thermostat.

Here is a technical note we prepared on the subject of troubleshooting an
overheating condition that may be of some help:

"Check first to see if you have a normal flow of cooling water coming out of
the exhaust. A normal flow of engine cooling water will usually be
sufficient to build up a head within the water lift muffler at idle to
overpower the exhaust pressure, and result in a "batching" of the water out
of the exhaust every second or so.

If you do not have a normal flow of water through the exhaust system and out
the back of the boat, your water pump could be too weak to provide
sufficient water flow; or you could have a restriction in a location that
can limit the total flow of water. Examples of these restrictions include a
blocked raw water through-hull, a piece of broken impeller in the elbow on
the outlet of the water pump, a clogged manifold, a blockage in the inlet
fitting to the exhaust system, etc.

If, on the other hand, you experience overheating while observing a normal
flow of water out of the exhaust, you will have to look for a restriction
that is preventing water from entering the block, and preferentially moving
through the by-pass hose instead. Examples of such a restriction are a
blockage within the "T" fitting, blockage in the diverter cap inside the
center of the water jacket side plate, or a thermostat that is stuck in the
closed position.

You can trouble shoot the extent of a blockage in this location, as well as
getting some short term relief, by clamping the bypass loop partially closed
with a pair of pliers or a small "C" clamp.

NOTE: The bypass loop is the hose between the "T" fitting in the center of
the water jacket side plate and the inlet to the thermostat housing. The
short hose on top of the head between the thermostat housing and the
manifold is not part of the by-pass loop.

If the flow of cooling water out of the exhaust reduces significantly as you
clamp off the by-pass hose, the restriction would be quite serious and will
probably require removal of the water jacket side plate and/or or the
thermostat for further inspection, cleaning, or replacement. In the case of
lesser restrictions, one of our customers was able drill an 11/32" hole
through the center of the "T" fitting and tap it with 1/8" pipe threads. He
was then able to connect a garden hose to pressure flush the "T" and
diverter cap to restore good flow. After flushing, an 1/8" pipe plug can be
installed in the hole in the "T" fitting."

Regards,

Don