Early last summer I wrote to you regarding my 86 Yamaha 150 overheating.
Thank you for the reply. (See Motor Overheating)
The solution ended up as follows...
I noticed something weird with the problem... The engine would not
overheat from hole shot to cruising at any speed as long as I went in a
As soon as I turned and straightened out again I lost water pressure. This
alone is probably normal but pressure would not come back.
The sacrificial anode had fallen off (it was not tightened properly). This
caused blisters in the paint on the lower unit. Once air flowed past
the blistered paint a "boundary layer" of air would stick to the
gear casing. No matter what I did (go faster, slower, turn again) water
would not break the air barrier and could not get into the grill.
After sanding off the blisters the problem was immediately fixed.
The boat ran great all summer. It took my friend with a Masters in
Mechanical Engineering to identify the problem.
Glad to hear your problem is solved. I've had this happen a couple of
times myself. To take it further we surmised that the air was only on the
low pressure side of the case. Hence it would switch sides in different
turns (direction). The intakes on Yamaha are not the best and the left
intake vents to the right and vice-versa. During the turn the intake
running in air would be the easiest one to pull from (air is lighter than
water) and the motor waterpump would pull air during the turn. When the
water flow resumed a nice steam pocket would form momentarily in the block
and then, instant overheat. In those instances we made water intakes that
rammed water into the gearcase at a pretty high pressure - enough pressure
that it would fill the side of the gearcase that was in the air stream. If
you have problems in the future that the contour won't cure, then try that
Anyway thanks for sharing your fix with us and hope your motor stays cool.
whatever questions you have to IHRTechTalk@yahoo.com.
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