Motors Trouble Shooting the Lower Unit By
FLC member Dick Newcomb
two most common problems with the lower unit (not
including a damaged prop) are the water pump and the
gear case. We'll cover the water pump first.
pumps on most outboards built after the 1940s, are
housings containing an impeller that is connected
to the motor's driveshaft. As the shaft turns, it
spins the impeller (looks like a fan) which creates
the suction to draw water in and force it up a copper
tube to the power head where it passes through interconnected
cooling cavities surrounding the motor's cylinders.
As the water circulates, it removes heat from the
combustion chambers, and then it is exhausted (underwater
in modern motors). As time passes, impellers, which
are generally made of a rubber material, get stiff
and worn. They gradually lose suction or some of the
blades may break off. Now, the motor will overheat.
Because the impeller is enclosed in the top of the
gear case, to service it, you need to drop the gear
case. That is, you need to unbolt it and slide it
down from the housing. In Johnson and Evinrude engines
this is accomplished by first disconnecting the gearshift
rod inside the housing (tower). There is a small oval
plate about half way up the side. Remove the two small
screws holding it on.
shift rod is connected by way of a brass coupler with
two bolts. Be very careful loosening and especially
retightening these bolts. The coupler is brass and
you can easily strip the threads. You really don't
want to buy a new one of these; they are expensive.
Disconnect the TOP one. You don't need to remove the
bottom bolt. Leave the coupler connected to the rod
going to the gear case.
you've disconnected the shift rod and the bolts holding
the gear case on, you can pull the lower unit off.
It may take a bit of a tug since the copper tube is
usually in a rubber gasket. Note: If you are working
on anything bigger than 40 hp, you may need two people;
they are heavy and awkward. You'll see the water pump
housing on top of the gear case with the driveshaft
running through it. Remove the screws and lift off
the top (Mercury outboards require a 'special tool'
for this procedure. I have these tools if you need
help.) Slide the impeller up off the shaft. There
is a small pin that holds it on the shaft. Be careful
this doesn't fall out and get lost. Note the blades
on the impeller: Have they taken a set (bend) leaning
backwards in the opposite direction in which the impellor
turns? Examine the water pump housing. If water has
frozen in the lower unit (a common problem), it may
be cracked or warped. In that event you'll need a
new one. Let's hope that didn't happen, and that all
you need to do is replace the impeller.
the new impellor on the shaft put the notch in it
over the pin. While you are doing that, turn the shaft
clockwise. This will cause the fins to seat (bend)
properly in the housing. Put the top of the water
pump back on and remount the gear case. It may take
some wiggling to get the copper tube back up into
the power head, but be patient. Reconnect the shift
coupler, remembering not to over tighten the bolt.
There is a notch in the shift rod that should match
up with the bolt shaft as it is inserted. Make sure
they line up before you tighten the bolt.
test your motor to see if it is pumping water well.
This all sounds like a difficult job but it really
isn't too bad. However, there is some BAD news --
Mercurys are a lot harder to work on, and many motors
don't have that troublesome but handy shift coupler.
Some require the removal of the power head from the
motor to drop the gear case, a major project. In those
cases you may want to work with someone who has worked
on those before, or pay the Marina. Ouch!
Evinrude warns that running a motor out of the water
even for a few seconds may destroy the impeller. This
is a little extreme but good advice. Always test motors
in a barrel of water, submerging the lower unit to
a depth equal to what it would be on the transom of
a boat. On more modern outboards, special 'earmuffs'
can be purchased to use with a garden hose, but I
don't trust them, and I rarely work on motors that
are some suggestions when working on any part of your
outboard: Have a container like a plastic food storage
container or small cardboard box handy to put all
your screws, etc. in. Be rigorous about saving parts
and labeling them, or better yet, take lots of pictures
as you go along. Invest in one of the many repair
manual guides available. Or the very best idea --
work with someone who has done this before.
Time: Servicing the gear case, OR: "This motor is
seized up!" Usually, this is NOT what's wrong.